Reviewed-by: Trust Me

- clear up some Q_OS_LINUX vs. Q_OS_UNIX
- clear up Q_WS_* vs. Q_OS_* (relatively theoretical exercise)
- use proper #if-#else cascades instead of free-standing #ifs

Most desktops allow popups to overlap the taskbar. We should allow this
additional space to be used by the completion popup.

Pop it upwards when it would otherwise go below the screen and shift it
to the left when it would otherwise exit the screen on the right.