Following mockups by Reda Lazri, the actual day it's marked by drawing a box
around the weekday and a 2px line on the cell belonging to the actual date.
Added icon to gnome-calendar.desktop. Tweaked css a little bit.
Added GcalEventView::will-delete to let thw window knows about the change.
GcalWindow will handle the hiding of the event-widget till the notification goes
away or the user click "Undo".
Added method in GcalView interface to retrieve and event-widget by it's uuid.
Added some cosmetic changes in GcalMonthView as well.