app: create custom provider for background colors

GcalEventWidgets get its background colors dynamically, so we are
creating a custom provider with a CSS class per calendar color, and
later on the proper class will be applied to the widget.
event-widget: use custom class for background colors
build: remove unneeded code

GObject type system zeroing instance private data, so initialization to
NULL and FALSE is not needed.
Misc code styles fixes.
comments: document utils functions, add FIXME
misc: whitespace and code style minor fixes
build: remove mention to non-existent file

Fix make distcheck
build: update GLib required version

g_strv_contains is a new acquisition of Glib, so we need to bump the
dependency up.
app:  make DBus-activatable

This commit installs a service file, renames the desktop file
to follow the bus name, and adds the DBusActivatable key to the
desktop file, following
https://wiki.gnome.org/HowDoI/DBusApplicationLaunching

Updates POTFILE files accordingly
ignore: updated