Add shortcut keys for indenting and outdenting lists to the list block

[BoardJames]

By default TinyMCE overrides the tab key to provide list indentation but stops tab from being used for navigation and makes it impossible to escape from the list block. So for Gutenberg we disabled the tab key.

For the list block this pull request enables meta+[ for list outdent and meta+] for list indent where meta means control on windows/linux and command on mac.

Edit: For locales without [ meta+m is used for indent and meta+shift+m is used for outdent.

This should resolve #1640

[coveralls]

Coverage decreased (-0.03%) to 17.786% when pulling e622210 on fix/1640-list-block-add-keyboard-shortcuts into 0f578ea on master.

[swissspidy]

This key combination is not really possible with non-US keyboard layouts I guess.

Edit: never mind.

[BoardJames]

Yeah, I should note my comment from the other thread:

One other thing that needs to be considered is that '[' and ']' are not on all keyboards. I'm told that the locales 'fr', 'nl', 'sv', 'ru', 'de', 'es' and 'it' do not have those keys. Apparently on textbox.io the square brackets, "meta+]" for indent and "meta+[" for outdent, are used on most locales and "meta+m" / "meta+shift+m" is used for the ones which don't have the square brackets.

[ellatrix]

Any idea what Google Docs does for keyboards without []? Are these keys not on these keyboards at all? A Belgian keyboard should be the same as French and it does have these keys.

How do we handle other key combinations for non Latin keyboards?

[afercia]

Any idea what Google Docs does for keyboards without []?

See screenshot 😬

[BoardJames]

Well I'm going to merge this and if any alternate better suggestions for shortcut keys come up it can be easily changed.