Add --filesystem=xdg-cache/thumbnails

Grant access to thumbnail cache folder in home directory.
This is safe as Gwenview already has access to everything (host) right
now as it's not fully portal aware yet.

Fixes: #168

org.kde.gwenview.json

@@ -8,6 +8,7 @@
     "finish-args": [
         "--device=dri",
         "--filesystem=host",
+        "--filesystem=xdg-cache/thumbnails:rw",
         "--filesystem=xdg-data/Trash",
         "--share=ipc",
         "--socket=cups",