fix: Make Tuba.Widgets.Avatar non-focusable in sidebar (#729)

Make Tuba.Widgets.Avatar non-focusable when used as a child button
of accounts selector, to prevent it from being selected during
tab-cycling.  Otherwise, it is cycled duplicate to the selector itself.

data/ui/views/sidebar/view.ui

@@ -39,6 +39,7 @@
 								<property name="popover">account_switcher_popover_menu</property>
 								<child>
 									<object class="TubaWidgetsAvatar" id="accounts_button_avi">
+										<property name="can_focus">false</property>
 										<property name="size">26</property>
 									</object>
 								</child>