views: move agenda view to main area

This prepares for multiple views to be there for mobile
users, and is the location at which a dedicated agenda
view for desktop will be, once the current agenda view
is refactored to suit desktop use-cases better.

Signed-off-by: Markus Göllnitz <camelcasenick@bewares.it>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/542>

src/gui/icons/calendar-agenda-symbolic.svg

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg height="16px" viewBox="0 0 16 16" width="16px" xmlns="http://www.w3.org/2000/svg">
+    <path d="m 1 1 v 2 h 14 v -2 z m 0 4 v 2 h 10 v -2 z m 0 4 v 2 h 14 v -2 z m 0 4 v 2 h 10 v -2 z m 0 0" fill="#2e3436"/>
+</svg>

src/gui/icons/icons.gresource.xml

@@ -2,6 +2,7 @@
 <gresources>
   <gresource prefix="/org/gnome/calendar/icons/scalable/emblems/">
     <file compressed="true">add-reminder-symbolic.svg</file>
+    <file compressed="true">calendar-agenda-symbolic.svg</file>
     <file compressed="true">calendar-month-symbolic.svg</file>
     <file compressed="true">calendar-week-symbolic.svg</file>
     <file compressed="true">calendar-today-symbolic.svg</file>