Minor improvements

• Moved towards Gtk.Application structure.
• Added docs pages for https://geraldjansen.github.io/hamster-lite/.
• Tweaked some shortcut keys (notably Enter same as Ctrl-R)

First usable version after refactoring

• Forked from https://projecthamster/hamster between v2.2.2 and v3.0
  and integrated many changes merged for v3.0 (up to Oct. 31, 2019).
• Removed DBus support and dependency (i.e. no more inter-application
  communication except through hamster-lite CLI).
• Simplified Storage object, limiting it to sqlite3 DB backend.
• Added simple json config file and removed gconf dependency.
• Removed translations of help pages (mostly outdated).