Add support for "enablement" property in plugin command contributions

[tsmaeder]

What it does

Reads the enablement property for commands contributed in a plugin package.json and updates toolbar items when necessary.

Fixes #12426

Contributed on behalf of STMicroelectronics

How to test

1. Install the attached plugin
2. Open the view "Test View Drag And Drop"
3. There are two toolbar actions: the left one prints the selction to the console and the right one changes the enablement of the other action
4. Click on the second action
5. Observe: the toolbar item becomes grayed-out/normal
6. Observe: you cannot execute the action if the item appears disabled and vice-versa.
7. Click on the "more" menu in the view and make the enablement in the menu is correct
8. Open the context menu on an item in the view and make sure the enablement is correct
9. Repeat the above tests in electron-native and electron-custom mode
10. Repeat the above tests in browser mode
11. Check main-menu behavior: in electron-custom and browser mode, enablement/disablement should work, in electron-native mode, main menu items should always be enabled (see Support dynamic menu items in electron #446). The "Selection" menu is a good candidate to check the behavior.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[tsmaeder]

Here's the test plugin:
custom-view-samples-0.0.1.zip

And here's the source:

tree-view-sample.zip

[planger]

Thank you Thomas! Works as advertised in browser, electron native and electron custom mode.
Code looks reasonable to me. I just added a minor nitpick inline.

[planger]

nitpick: maybe it helps readability if we extract two variables with helpful names for each of the two parts of the conditions.

[tsmaeder]

@planger I changed the isEnabled() logic. Have a quick look?

[planger]

Thank you, much better imho!

[vince-fugnitto]

@tsmaeder @planger we may have to revert this change, it breaks context-menus in electron.
For example: #12548.

[tsmaeder]

@vince-fugnitto since action enablement works just fine for other context menus (file explorer or the general context menu on the editor), this might just be a bug in the action enablement that is revealed since we now actually honor the enabled state.