Nintendo DS screen layout switcher

A basic screen layout switcher. It just sets desmuume_screens_layout option, so it only works with DeSmuME and DeSmuME 2015 cores, but not MelonDS or any potential other cores. When GNOME/retro-gtk#15 is fixed, we can easily change it to offer other options and to make it work with any core.

Screenshots:

Needs icons!

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Update: @aplazas did the icons, and we've removed popover in favor of just linked buttons:

Also added right/left in addition to left/right.

Fixes a part of #13 (closed)

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Update: back to popover, added quick switch, keyboard and gamepad shortcuts.

Keyboard shortcut depends on GNOME/retro-gtk!30