year-view: Mark working days according to locale

Currently, on the year view, Saturday and Sunday are marked
as non working days by default, despite the Locale settings and the
fact that in some Countries Sunday and/or Saturday are workdays.

This patch introduces a new function "is_workday" to check
whether a day is a working day according to the territory part of
Locale.

https://bugzilla.gnome.org/show_bug.cgi?id=746927

data/theme/gtk-styles.css

@@ -89,7 +89,7 @@ calendar-view.offset {
     font-size: 9pt;
 }
 
-.year-navigator.sunday {
+.year-navigator.non-workday {
     color: alpha(@theme_fg_color, 0.5);
 }
 
@@ -132,7 +132,7 @@ calendar-view.offset {
     background-color: @theme_selected_bg_color;
 }
 
-.year-navigator.with-events-sunday {
+.year-navigator.with-events-non-workday {
     background-color: alpha(@theme_fg_color, 0.5);
 }