I/3287: Table and table cell properties plugins

[jodator]

Suggested merge commit message (convention)

Feature: Introduce table properites conversion. Closes ckeditor/ckeditor5#6048.

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PR constallation

Build on ckeditor5/i3287 branch:

Edit:
the build fails because watchdog detects StylesConverter as "connected" https://github.com/ckeditor/ckeditor5-watchdog/blob/73844f37f90ea0bf0e0a756752c62b1ce86e334b/src/watchdog.js#L452 because of the StylesConverter singleton... it is fixed by making StyleConverter a non-enumerable property.

This PR requires changes:

• engine: I/3287: Styles normalization (part of table styles) ckeditor5-engine#1807 (comes with own constellation of changes)
• ui: I/3287: Temporary PR - add "UI" type of list dropdown ckeditor5-ui#528 (might be reverted as UI is not a part of MVP)

Description

This is a draft PR to start review process of this behemoth change. This PR will need at least two reviewers and probably more then 2 passes.

At this stage I'd like to review the general idea behind table styles:

• Changes in the engine that enables styles normalization (separate PR)
• Table properties model, conversion, desired output.

Conversion (* - means any value but can be constrained by Styles normalizer):

1. TableProperties handles:
   • borderWidth=*, borderColor=*, borderStyle=* (<table style="boder-top:1px solid blue;...">):
   • backgroundColor=* (<table style="background-color:some-color">)
   • width=* / heigh=* ( <table style="width:xx">)
   • alignment=left|right|center (<table style="margin-left:0,margin-right:auto"> - as in MDN, upcasts also <table aling="left|right|center">).
2. TableCellProperties handles:
   • borderWidth=*, borderColor=*, borderStyle=* (<td style="boder-top:1px solid blue;...">)
   • backgroundColor=* (<td style="background-color:some-color">)
   • padding=* (<td style="padding:2em">)
   • verticalAlignment=* (<td style="vertical-align:top">) - should be fixed to
   • horizontalAlignment=left|fight|center|justify (<td style="text-align:left|right|center"justify">)
3. TableColumnRowProperties handles:
   • heigth=* (<td style="height:*">)
   • width=* (<td style="widith:*">)

[jodator]

@Reinmar This PR implements the two main plugins:

1. TableCellProperties - glues *Editing and *UI parts.
2. TableProperties - glues *Editing and *UI parts.

From that I've implemented 16 commands:

For table properties:

• tableBorderColor - manipulates a table's borderColor attribute
• tableBorderStyle - manipulates a table's borderStyle attribute
• tableBorderWidth - manipulates a table's borderWidth attribute
• tableAlignment - manipulates a table's alignment attribute
• tableWidth - manipulates a table's width attribute
• tableHeight - manipulates a table's height attribute
• tableBackgroundColor - manipulates a table's backgroundColor attribute

For table cell properties:

• tableCellBorderStyle - manipulates a tableCell's borderStyle attribute
• tableCellBorderColor - manipulates a tableCell's borderColor attribute
• tableCellBorderWidth - manipulates a tableCell's borderWidth attribute
• tableCellHorizontalAlignment - manipulates a tableCell's horizontalAlignment attribute
• tableCellWidth - manipulates a tableCell's width attribute
• tableCellHeight - manipulates a tableCell's height attribute
• tableCellPadding - manipulates a tableCell's padding attribute
• tableCellBackgroundColor - manipulates a tableCell's backgroundColor attribute
• tableCellVerticalAlignment - manipulates a tableCell's verticalAlignment attribute

Please validate if the names are OK (especially if the attribute name is OK ie borderColor vs tableBorderColor). Eventual change would be 20 min tops, so thats not an issue ;)

Most of the commands code is done in base commands which at least in MVP do the same: get a table or cells from selection and apply or remove attribute. If command operates on top/right/bottom/left value object it will set command.value only if all four are the same.

As a simplification (which also works) I've made that command sets one string value in the model instead of top/right/bottom/left object. This can be changed in follow up.

[oleq]

LGTM. I pushed documentation fixes straight to the branch because otherwise, GH shows me Unicorns when submitting.

[jodator]

@oleq done. Please note that I've added small addition to the docs in the engine so this PR is needed also: ckeditor/ckeditor5-engine#1816.