shell: Make PanelConfigView dodge other panels
- shell: Don't set panelconfigview as an applet for stacking order purposes
This was added as a wokaround for the case of overlapping other panels.
As well as being semantically not entirley correct, it also causes issues with stacking order as other popups from the panel that should be on top of this window.
- shell: Make PanelConfigView dodge other panels
This uses a temporary non-public API to limit the screen region used by the popup placement, to avoid overlapping with other panels.