month-view: initial work to new mockup from Lapo

There's a bunch of stuff missing here, but this code is already working.
This includes only part of the drawing code, I already take into account
here LTR and RTL languages.
Here month-view make use of classes for styling different regions of the
grid, there are:
1. header class, which is used for the font and color details of the
days naming on the top of the grid
2. current class, which is used for drawing the current day number and
its background
3. lines class: which set the color property of the lines in the grid
4. :selected pseudo-class is used for drawing the font on the days
marked when creating an event. This will work as well for keyboard
navigation

What's missing here:
1. properly update click/press marks to new model
2. draw border around the selected days

data/theme/gtk-styles.css

@@ -26,15 +26,24 @@
     font: 10;
 }
 
+.calendar-view.header {
+    font-weight: bold;
+}
+
 .calendar-view.current {
-    background-color: alpha(@theme_selected_bg_color, 0.2);
-    color: @theme_selected_bg_color;
+    background-color: black;
+    border-radius: 2px;
+    color: white;
+    font: bold;
 }
 
 .calendar-view:selected {
     color: @theme_selected_bg_color;
-    background-color: shade (@theme_base_color, 0.9);
-    font: bold;
+    font-weight: bold;
+}
+
+.calendar-view.lines {
+    color: alpha(@theme_fg_color, 0.4);
 }
 
 .event-widget {