date-selector: initial implementetation

No functionality is ready, only project settings and class type.
date-selector: load popover with date selector
edit-dialog: fixed wrong setting of date selector sensitivity
date-selector: set max width for date entries

Fixes those giant horrible entries that were being cutted off of the dialog.
date-selector: set calendar's date
date-selector: shows parsed date at the date's label

Also, GcalDateSelector do it's best to find out day, month and year relative positions at the date mask.
date-selector: ajust entries' names
date-selector: setup date entries' text

They still are useless.
date-selector: limit number of digits of entries
date-selector: validate entries' input
date-selector: allow calendar to select the date

For testing reasons, we don't fire MODIFIED signal yet.
date-selector: clear code, improved set_date func
date-selector: clamp day & month values before validating it
edit-dialog: link date & time selectors
date-selector: set date on GtkEntry::focus-out-event
date-selector: update date on GtkEntry::activate signal
date-selector: fix focus-out propagation issue
date-selector: fire MODIFIED signal

Also, some code cleanup.
date-selector: show Day, Month and Year labels according to it's position
edit-dialog: fix sizing issue