[Nested widgets] Drag and drop of nested block widgets

[Olek Nowodziński]

Part of #10931.

At the moment there's no way to drag and drop any block widget within nested editable because "drop lines " are not shown in nested editables at all.

[Piotrek Koszuliński]

I made it a block widgets only ticket. Inline widgets dnd will be handled in different ticket.

[Piotrek Koszuliński]

#12072 - the same case - inline widgets.

[Olek Nowodziński]

Pushed changes and tests to ​branch:t/12006:

• Drag and drop of block widgets is enabled for nested editables.
• Drag and drop considers filters (allowedContent). If the widget is disallowed in some nested editable, there's no chance it's dropped there (no drop line is displayed).
• Prevented pathological drop into widget's own nested editable.

[Piotrek Koszuliński]

This is definitely a ticket for major release, because it changes the behaviour of dnd. Prior to 4.5 nested widgets are not enabled (dnd, insertion etc). Enabling dnd in 4.4.2 would suddenly unlock the ability to nest widgets and that's not safe for a minor release.

[Piotrek Koszuliński]

I reported followups for this ticket: #12091, #12090.

[Piotrek Koszuliński]

R+. Doesn't work on IE8 but we don't plan to have nested widgets support in IE8 yet, so it's ok.

[Piotrek Koszuliński]

Remember to rearrange commits in the branch so tests go after dev.

[Olek Nowodziński]

​git:541f71b landed in major