Add plugin list in settings page.

[Mansarde]

Inspired by #3 I added two collapsible lists in the settings page: Installed - shows all plugins installed via Galore. History - shows missing plugins that were previously installed via Galore. Entries in History can be removed individually or in bulk.

In the backend, a new key "history" is added to the data.json which holds the array of history elements.

I made this mainly for myself, but maybe it's of use to someone else. Let me know if there's any changes you'd like me to make. :)