calendar-button: Add mnemonic to calendar label

Mnemonics are useful for navigating through the interface more easily,
besides improving the accessibility. Since the calendar button is an
important button for using the application, it is interesting to have
a mnemonic associated with its label.

Using the letter C for this label seems reasoable, since there is no
conflict with the labels simulteneously active in the application, and
it is easy to remember.

src/gui/gcal-calendar-button.ui

@@ -20,7 +20,8 @@
                   <object class="GtkBox">
                     <child>
                       <object class="GtkLabel">
-                        <property name="label" translatable="yes">Calendars</property>
+                        <property name="label" translatable="yes">_Calendars</property>
+                        <property name="use-underline">True</property>
                       </object>
                     </child>
                     <child>