KDE: Update KDE Plasma to 6.3.0

Announcement: https://kde.org/announcements/plasma/6/6.3.0/

kde.mk:
 - Bump kdecoration library version.

devel/plasma6-plasma5support:
 - USE_KDE+= idletime

x11-wm/plasma6-kwin:
 - Backport upstream fix for kwin_x11 crash.

www/plasma6-plasma-browser-integration:
 - Do not build flatpak-integrator, which is no-op for us
   and fails to build due to missing headers.

sysutils/plasma6-kinfocenter:
 - Add build dependence on graphics/vulkan-headers and pkgconfig.

Special thanks to Loïc Bartoletti <lbartoletti@> for tracing
clang/c++/Qt culprit, and to Jason E. Hale <jhale@> for prompt fix.