kcm/tabbox: Make the preview show desktop if the option is selected

The thumbnail that represents the "desktop" in the switcher preview
is taken from the default wallpaper, and it will update automatically
as it changes in future versions of Plasma.

The current one is also added along the other thumbnails as a fallback.

BUG: 309401
FIXED-IN: 5.24
2 jobs for !1718 with work/iasensio/tabbox_preview_desktop in 19 minutes and 29 seconds (queued for 11 seconds)
latest detached
Status Name Job ID Coverage
  Build
passed freebsd_qt515 #166688
FreeBSD

00:08:01

passed suse_tumbleweed_qt515 #166687
Linux

00:19:29