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When the button icon was an XfcePanelImage, it needed to be reloaded
through the size_changed() function whenever the icon theme changed.
That is not needed now that it is an GtkImage with an icon name.

The check was supposed to allow using Super_L as a shortcut, so that users
could also have Super_L + other keys launch things and Whisker Menu wouldn't
pop up. That hasn't worked ever since Xfce switched to Gtk3.

The check was also supposed to force other menus closed before popping up
Whisker Menu. That also no longer works after the switch to Gtk3.

The grab check now serves no purpose.

This way, when I change a virtual function I will know immediately
if I have forgotten to update any overrides of the base function.

Moving the window outside of the panel space stops the panel from
shifting it over once it is shown. This does mean that the window
border no longer overlaps the panel edge like a regular Gtk menu.