Implement "take a screenshot" feature

* Like in android - press power+volume down, screenshots are saved
  in /sdcard/Pictures/Screenshots (if /sdcard is mounted) or /tmp

Change-Id: Iaefa15b11a1d5fdfac57d77388db1621f378a8d4
Signed-off-by: Vojtech Bocek <vbocek@gmail.com>

Conflicts:
	gui/action.cpp
	gui/devices/1024x600/res/ui.xml
	gui/devices/1024x768/res/ui.xml
	gui/devices/1280x800/res/ui.xml
	gui/devices/1920x1200/res/ui.xml
	gui/devices/2560x1600/res/ui.xml
	gui/devices/800x480/res/ui.xml
	twrp-functions.cpp
	twrp-functions.hpp