Move dialog - Show tree structure inside the content selector

[alansemenov]

By default, content items inside the content selector should be tree-structured with the same order as in the Content Grid. When the filter applied, the selector should switch to flat structure (like now).

The core of this task is to implement a new class DropdownTreeGrid which will then be used inside the combobox (if certain config parameter is specified).

[alansemenov]

Great job! Some things to fix:

1. The first time I open the dropdown, scrolling is not working. If I collapse and expand again, it works fine.
2. If I search by keyword, then select a node, then click "x" to delete selection, the keyword remains in the field. We should clear it after clicking "x".
3. Would be convenient to auto-expand the tree down to selected item, and highlight selected item in the dropdown. This will help a lot if I want to move the item one level up, for example, or to a sibling folder.

[sshklianko]

Could not reproduce 1. even with browserstack emulation mode/
2. Is implemented now.
3. I did not add it yet - let's create separate task for that as this task has already taken lots of time.