ToolTipHandler.cs: force tooltip to be above the Top Most windows every second.
Utils.cs: fix transparent BitmapButton did not trigger tooltip in ZKL Linux

Also... refactored (rename) 3 variable in GetHoveredControl() for less confusion ("parentControl" -> "thisControl" because it doesn't always contain a parent control)
Minor UI tweak (such as rename ".. custom singleplayer game" -> ".. custom offline game")
("spring://extra" -> "spring://extra/skirmish" is also just a visual tweak)