[Fuze+][Touchpad] Improve touchpad power managment

- take advantage of the new rmi power function implemented to:
1) lower usual power_mode to low_power as it seems to be enough and might save
some battery
2) implement a system that lower that state to very_low_power
after 1 minute of inactivity.
3) implement touchdev_enable(bool) that can be use later to disable the
touchpad when needed
4) improve the debug screen report of the current power state and
changing the power state using volume keys

Change-Id: I0b372696d4b2bef5360c778d0500870fd9badee1
Reviewed-on: http://gerrit.rockbox.org/525
Reviewed-by: Amaury Pouly <amaury.pouly@gmail.com>

firmware/target/arm/imx233/sansa-fuzeplus/button-target.h

@@ -24,6 +24,7 @@
 #include <stdbool.h>
 bool button_debug_screen(void);
 void touchpad_set_sensitivity(int level);
+void touchdev_enable(bool en);
 
 /* Main unit's buttons */
 #define BUTTON_POWER                0x00000001