i18n: Add context for Add Calendar buttons

Due to some languages using the "nplurals=1; plural=0" plural form,
and the string already in use in src/gui/calendar-management/gcal-new-calendar-page.c
with g_dngettext, add context to the other "Add Calendar" strings to fix the problem.

AFAIK, other locales (confirmed: be_BY, fa_IR, id_ID, kk_KZ, ko_KR, my_MM, zh_CN)
are also affected by this issue.

src/gui/calendar-management/gcal-new-calendar-page.c

@@ -162,7 +162,7 @@ update_add_button (GcalNewCalendarPage *self)
     }
   else
     {
-      add_button_label = g_strdup (_("Add Calendar"));
+      add_button_label = g_strdup (C_("button", "Add Calendar"));
     }
 
   gtk_button_set_label (GTK_BUTTON (self->add_button), add_button_label);

src/gui/calendar-management/gcal-new-calendar-page.ui

@@ -190,7 +190,7 @@
                 <child>
                   <object class="GtkButton" id="add_button">
                     <property name="halign">center</property>
-                    <property name="label" translatable="yes">Add Calendar</property>
+                    <property name="label" translatable="yes" context="button">Add Calendar</property>
                     <property name="sensitive">False</property>
                     <signal name="clicked" handler="on_add_button_clicked_cb" object="GcalNewCalendarPage" swapped="no" />
                     <style>