message-toolbar: Don't make stack vertically homogeneous

Otherwise it forces the composer to be bigger than needed on narrow
screens because of the text on other stack pages.

src/session_view/room_history/message_toolbar/mod.blp

@@ -21,6 +21,7 @@ menu message-menu-model {
 template $MessageToolbar: Adw.Bin {
   Gtk.Stack main_stack {
     transition-type: crossfade;
+    vhomogeneous: false;
 
     Gtk.StackPage {
       name: "composer";