Add GsPluginJobUninstallApps and port uninstalling apps to the new threading model

As with !1927 (merged), this is an alternative to the first few commits of !1875.

The main differences from !1875 are the split between install/uninstall as separate jobs, and the terminology rename of ‘remove’ to ‘uninstall’ because it’s the antonym of ‘install’.

If/When this lands, I can then start rebasing and landing the rest of !1875.

Signed-off-by: Philip Withnall pwithnall@gnome.org

Helps: #1472