WIP: Adaptive ui

This is an effort to change the UI to be adaptive for use on mobile devices. It aims to implement this mockup

So far mainly the .ui files have been touched.

Known problems/Open questions:

• When the leaflet starts folding, some visual glitches appear (constantly switching between folded/unfolded view)
• Have round buttons for the dates in GcalSmallView (should it be a button at all? What should happen if it's clicked? Scroll to the events of that day?)
• Indicate the presence of events in the date grid by small coloured circles
• Have the "current time" indicator before the next event (check how that line is drawn in GcalWeekCell?)
• Use "today" "tomorrow" in the headers instead of the weekday (are there translated strings already available somewhere?)
• What should happen when a event is clicked in the sidebar? Edit the event I suppose?
• "Today" button is missing (both in the implementation aswell as in the mockup in the GcalSmallView
• What about the "yearly" view? Still needs design?
• Today/forward/backward buttons are currently kind of ugly in the headers of GcalWeekView/GcalMonthView/GcalYearView

Main To do:

• Add specialized views for the sidebar (ish) - works for both Week and Month view!
• List events in the sidebar/specialized views (polish needed)
• Move today/forward/backward buttons into separate header
• Cleanup comments/whitespaces (Remove all unused/unnecesary copy pasta)
• Make it pretty (beauty is in the eye of the beholder!)

Edit: I will cleanup the commit messages at some point. Currently I'm experimenting too much to have a clean commit history.

PS: I'm still pretty new to GTK, so please be gentle ;)