gtkfilechooserwidget: set 'New folder' button focus-on-click

As general rule, all buttons that launch a menu should
not grab focus on click, because otherwise when the menu
is closed the focus goes back to the button instead of
the previously focused widget, which is the one the user
was interacting with.

So this is also the case for the 'New Folder' button on
the filechooser.

Fixes #2557

gtk/ui/gtkfilechooserwidget.ui

@@ -68,6 +68,7 @@
                                   <object class="GtkMenuButton" id="browse_new_folder_button">
                                     <property name="tooltip-text" translatable="yes">Create Folder</property>
                                     <property name="use-underline">1</property>
+                                    <property name="focus-on-click">0</property>
                                     <property name="popover">new_folder_popover</property>
                                     <signal name="notify::active" handler="new_folder_popover_active"/>
                                     <child>