Automatically close fds for inert resources

If the plasma window resource is inert, the associated get_icon handler
won't be invoked. This results in leaked file descriptors.

With this change, the corresponding close() function call will be
generated to avoid leaking file descriptors and potentially crashing the
session due to kwin running out of fds.

BUG: 438097
1 job for !299 with work/zzag/close-inert-fds in 8 minutes and 5 seconds (queued for 47 minutes and 37 seconds)
latest detached
Status Name Job ID Coverage
  Build
passed linux kf5-qt5 qt5.15 #133855

00:08:05

60.0%