PID tuning page improvements & ideas

[bkueng]

Follow-up to https://github.com/mavlink/qgroundcontrol/pull/9256:

• disable graph over telemetry link (e.g. if update rate is < 20Hz)
• keep data when switching between axis (graph + clipboard)
• If the roll/pitch/yaw radio button setting is remembered when I leave the PID tuning view and later come back.
• add ability to lock roll+pitch sliders together
• start graph on arming if not already running
• mouse zooming might be useful
• potentially show MC_ROLLRATE_MAX in the graph
• With a button that saves a screenshot the current plot (whether it's stopped or not). I was taking screenshots manually.
• If a horizontal line at 0 deg/s was visible in the plot at all times.
• add support for fixed-wing

[mortenfyhn]

Another, not super important thing: On Linux, the graphs/sliders scale poorly if the QGC window is tall and narrow.