[Util] Added LineEditButtonManager.
[Util] Use LineEditButtonManager in ClearLineEditAddon.
[Util] Added manager-aware ClearLineEditAddon ctor.
[Util] Fixed docs.
[Util] Code style.
[Util] Don't update minimal size hint in LineEditButtonManager.
[Util] Button for invoking tags selector in TagsLineEdit.

For #1603.