Client: Show message box if launch fails

If engine launch fails due to any exception, show a nice message
box with the error message before quitting.
Client|UI: Reduce game menu's height if task bar visible

It looks messy if the game menu overlaps the task bar. While the game
menu remains centered vertically, its height will now be reduced to
avoid overlapping the open task bar.
Client|UI: Improved popup and popup menu behavior

When a popup widget is closing/closed, no events will be dispatched
to its children, i.e., the content widget.

The highlight of a popup menu is now cleared immediately when the
menu is closed.
Merge branch 'master' of ssh://github.com/skyjake/Doomsday-Engine