switchMonitor: switch to next config upon initial keypress

In GNOME-3.24, pressing Super+P or a similar function key would cause a switch to the next available monitor configuration.

However, in GNOME-3.26, this was reimplemented in mutter and gnome-shell and the behaviour is now different: pressing Super+P and releasing will cause no change in montor configuration[1]. In this new design you have to press Super+P and keep holding Super in order to keep the switcher open, then press P again (or use the arrow keys or mouse) to select the next one in the list.

This is incompatible with many Asus products such as Asus X530UN, where pressing the presentation mode media key (Fn+F8) actually generates the following keypress events from the keyboard controller:

Fn pressed: nothing F8 pressed: nothing F8 released: Super press, p press, p release, Super release (quick burst) Fn released: nothing

With this firmware behaviour it's not possible to hold the keys and have the dialog come up so that you can select another new mode.

To solve this, when the switcher is opened, select the next available display config by default, which is more similar to the pre-GNOME-3.26 behaviour. Now pressing Fn+F8 on this laptop will result in the display mode switch taking place.

[1]: The mentioned desired behaviour will at least happen after mutter#281 (closed) has been fixed