Cam control using gpio functions instead of fake LEDs

An attempt to solve #596 

Instead of abusing the LED defines, with this proposal you would use e.g. `<define name="DC_SHUTTER_GPIO" value="GPIOC,GPIO12"/>` instead of `<define name="DC_SHUTTER_LED" value="7"/>` with LED_7 being defined appropriately somewhere else...

Maybe we should introduce additional defines like `#define PA1 GPIOA,GPIO1` which would enable you to write `gpio_set(PA1)` instead of `gpio_set(GPIOA, GPIO1)` and also make the definition of gpios for things like DC_SHUTTER_GPIO easier (could also be used for LEDs).