CMakeBuildDirChooser: Set 'OK' as default button

Respect KDE HIG.
Fix gdb version detection.

Whether I don't get regexp's at all, or it was really a weird (and even
broken) way to check the gdb version.

BUG: 334114
FIXED-IN: 4.6.1
Merge remote-tracking branch 'origin/4.6'