Adaptive preferences window

The second part of !153 (merged).

• Use HdyLeaflet for sidebar
• Fork GtkStackSidebar as PreferencesSidebar, and use it for sidebar

Screencast_from_12-09-18_15_19_51

Are we the first app outside of libhandy example to ever use HdyTitleBar? :D

TODO:

• Some labels should probably be wrapped or ellipsized, like the "Press key on your keyboard" one Done in a separate MR
• It currently bundles CSS styles from libhandy example. Most of these were merged into GTK 3.24.1, so we'll need to drop them when it's available in the SDK
• Since subpage is being kept around after closed (to ensure that slide transition works, it's only removed when opening another one), it eats keyboard events. This will need to be fixed Fixed by only keeping it around until the end of transition.
• Split into more commits
• Maybe split into multiple MRs
• Scrollbar in the left pane is weird Also an issue in libhandy example. See libhandy#37 and libhandy!110 Fixed both here and upstream
• There's a rare crash when closing the window (cannot reproduce anymore)
• (org.gnome.GamesDevel:2): GLib-GObject-WARNING **: 13:34:44.635: g_object_set_is_valid_property: property 'fold' of object class 'HdyLeaflet' is not writable
• libhandy#42
• libhandy#56

Partially fiхes #80 (moved)