date-chooser: add date chooser widget

This is mainly a copy-paste from an old branch authored
by Matthias Clasen. The code style was updated to match
Calendar's one, and the theming was improved a little bit.

data/Makefile.am

@@ -51,10 +51,12 @@ EXTRA_DIST=                     \
   shell-search-provider-dbus-interfaces.xml \
   ui/alarm-row.ui               \
   ui/calendar-row.ui            \
+  ui/date-chooser.ui            \
   ui/date-selector.ui           \
   ui/edit-dialog.ui             \
   ui/help-overlay.ui            \
   ui/menus.ui                   \
+  ui/multi-choice.ui            \
   ui/quick-add-popover.ui       \
   ui/search-view.ui             \
   ui/source-dialog.ui           \