Fix black background w/ GtkPopover and Unity/Ambiance: Bug #735421

If a GtkScrolledWindow is present in a GtkGrid (or perhaps just a
GtkScrolledWindow) which are in a GtkPopover, Unity/Ambiance will
set the entire grid's background color to black.  Problem corrected
manually in the code.

./configure --enable-unity must be used to activate this fix.
Allow for non-date-specified Quick Add: Bug #735939

Unity/Ambience theming bug led us not to show any GtkPopovers that
are attached to the GtkHeaderBar, instead using the current day for
Ctrl+N (or the Quick Add button).  That regressed functionality by
not allowing for users to enter a Quick Add event devoid of a
starting date.  This restores that functionality, using a DeckWindow
instead of a DeckPopover on Unity/Ambiance.
Don't set end date for single-date Quick Add: Bug #735940

If the user Quick Adds a recurring event by clicking on a date,
prior code would use that date as the start and end date of the event,
which screwed up the RRULE.  Now a single day is only treated as
the starting date of the recurring rule.

Additionally, if a date is selected and a weekly recurring event is
Quick Added that starts on a different day of the week, the original
start date is dropped (for consistency).  In other words, clicking on
a Wednesday and entering "Meeting every Tuesday" will generate an
RRULE that starts on the upcoming Tuesday.
Treat Quick Add as create, not update, event: Bug #735853
Auto-enable event's start day for new weekly RRULE: Bug #735834
Properly handle modified master and generated instances: Bug #735946

Modified master instances must look for situation where their RRULE
was dropped (and hence all their generated instances must be dropped).
Modified generated instances must only update their instance in
memory and none other.
Show recurring explanation in event editor
Merge branch 'master' into wip/732930-explain