HUGE: generic input relative mode implementation

HID plugin changes reverted. Relative settings moved in the Profile editor.
Relative values handling code has been moved into QLCInputSource class.
QLCInputSource class has been turned into a QObject/QThread subclass. Each input source
in relative mode starts up a thread to emit synthetic values.
VCWidget sends input values and 'feedbacks' to its input sources in relative mode
to keep in sync with the external controller and the UI status.
Reviewed a few includes all over QLC+.

Note: QLCInputSource must now be used as a pointer and not as a static class.

TODO: update profile editor documentation