Re-enable mouse hover detection over insets. This was disable at rev 17589 to fix...

Re-enable mouse hover detection over insets. This was disable at rev 17589 to fix bug 3144 which has been re-opened for this.
There is a user visible change with this commit: For text insets, the button stays "highlighted" as long as the mouse cursor is within the inset but not within a nested inset. Formerly the button was highlighted only when hovered. This new behaviour is better because we can now imagine funkier behaviour like inset color background change when hovered. Also the small math pink corners can be adapted to react to mouse hover in the future. Ideally we should also "highlight" the button when entering the inset via the keyboard or when the keyboard cursor is just before the inset (because we can then use Ctrl-i).

* BufferView:
  - getCoveringInset(): new method.
  - workAreaDispatch(): use getCoveringInset() instead of LyXText::checkInsetHit()

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@17610 a592a061-630c-0410-9148-cb99ea01b6c8