core: Demote image formats' priority from preservation in clipboard manager

Most usually, applications either expose clipboard content either as text
or as images, so the prioritization here is pointless. However there's some
outliers like LibreOffice Calc which exports content as both image and text
formats (besides other internal ones).

In that mixed case, we probably prefer to keep text formats, rather than
image based ones.

Closes: https://gitlab.gnome.org/GNOME/mutter/issues/919