ZKL: final tooltip fix

Changes:
1) always force tooltip to reorder its z-order (will fix tooltip behind ZKL as long as nobody is doing spam competition to make ZKL the TopMost position)
2) fix tooltip not appearing for buttons in Linux (GetHoveredControl was skipping transparent button in Linux).
3) rename tooltip: "singleplayer game vs AI" -> "offline game vs AI" for Skirmisher tab button