clients: configure cursor theme from XCURSOR_* env

If XCURSOR_THEME or XCURSOR_SIZE is set, use it as the cursor theme
or cursor size.

This is similar to what Qt and some X11 apps do.
1 job for xcursor-env in 5 minutes and 41 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #39029
build-native

00:05:41