Improve support for widget's selection in editor

[msamsel]

Type of report

Feature request

Provide a description of the new feature

There bunch of bugs in the editor which are mostly the result of partially selected widgets. The behaviour might vary between browsers. For example, in Chrome text inside widget cannot be selected, where for FireFox it's possible if selection starts before widget.

This kind of selection cases which contains only partially selected widget triggers bugs in various places, which are fixed individually. Maybe there could be some common mechanisms, which will prevent the situation of the partially selected widget. It could also allow to truly treat widget as a single entity during selection, which cannot be partially selected.

This issue might be treated as a hub for smaller bugs detected during the editor usage. The most problematic cases are usually operations which rely on the selection or ranges. Such as inserting/removing content, clipboard operation, undo, etc.

[msamsel]

Another case of how it interferes with image2 under FF:

[msamsel]

Might be somehow related: #3698, #3699

[f1ames]

There is already related PR, however it concerns widgetselection so the cases when widget is on the beginning/end of the content only - #2866.