Lose libpeas dependency

[jeremypw]

Fix #1498

• Use gmodule-2.0 directly
• Implement only the necessary parts of libpeas in PluginManager (based on the equivalent class in io.elementary.files)
• Update code style a bit, add some headers
• Move some code into PastbinDialog.vala where it is actually used

NOTE: All the plugins are loaded and made resident in the same way as Files. They are activated and deactivated (but not unloaded) as required. When deactivated the interface signals are disconnected.

Need to check all plugins still work as expected.

[jeremypw]

Closing in favor of migrating to libpeas-2 for now.