Refactor the plugin system to not use libpeas

[jeremypw]

Problem

See #1497 (comment)

Proposal

For ease of porting to Gtk4 and to remove an unnecessary dependency we should implement the plugin system in the same way that Files does

Prior Art (Optional)

See the relevant code in Pantheon Files.

In Code it will be necessary to implement activating and deactivating individual plugins and adjust the PreferencesDialog accordingly as in Files there are only core plugins that are always active.