Param meta description

[LorenzMeier]

@bendyer Phillip has hand-crafted a param meta XML file as a stopgap and I'm injecting it for PX4:
https://github.com/PX4/Firmware/blob/master/Tools/parameters_injected.xml

This gets picked up by the QGC UI.

The param tools to generate this are here:
https://github.com/PX4/Firmware/tree/master/Tools/px4params
I plan to factor them out to a separate repo.

The comment style which gets parsed is this:
https://github.com/PX4/Firmware/blob/9b45daa8f7fac2172e6b4ec0204b0b8b98b82137/src/modules/attitude_estimator_q/attitude_estimator_q_params.c#L44-L52

I'm open to either extend this or to use the XML format as the main format. Right now we're doing the slightly absurd thing of parsing the params from C files into XML, then generate C++ out of it and never compile the C files. That's a transitional artefact though.

/**
 * Complimentary filter accelerometer weight
 *
 * @group Attitude Q estimator
 * @min 0
 * @max 1
 * @decimal 2
 */
PARAM_DEFINE_FLOAT(ATT_W_ACC, 0.2f);