Refactor GamepadMappingSet
Rename GamepadMapping as Mapping and GamepadMappinSet as MappingSet
Add headers to Vala files
Ignore files produced by compilation
Split gamepad-input-type from gamepad
Split mapping-operation-type from mapping
MappingSet: Treat line reading errors while constructing from file
Gamepad: Allow to automatically get its mapping at construction time
MappingOperationType: Add apply()

This adds the apply method the MappingOperationType to allow to easily
apply a giver operation to a value to adapt.
gamepad-mapping: Correct the "Microsoft X-Box 360 pad" mapping

The trigger operation was applied to the triggers input, which is wrong
as they expect regular trigger values.