MediaWiki theme: Polish the toolbar design

This solves some of the issues with toolbar, by adding hover states
to tools, better separation of tools, adding anchors to dropdowns so
you can see the parent menu, and making action buttons flush.

New dropdown indicators and undo redo icons + alignment fixes in two icons on toolbar.
min width to the dropdown so more|less will be stable.

Bug: T86693
Change-Id: I32d5e094c83f6c704672dd2cb52517717141366f