[MSFS] Read out com volumes

[ltoenning]

Currently we do not read out the com volumes for MSFS.

From https://github.com/swift-project/pilotclient/issues/100#issuecomment-1015823687:

For MSFS2020 the volumes are COM VOLUME variable with an index specifying the radio in question:

(A:COM VOLUME:1, Percent) (A:COM VOLUME:2, Percent)

These are part of the SDK in MSFS2020.

[tigert]

Just for the sake of completeness: On MSFS2020, the COM1 / COM2 transmit / receive functionality already works, also COM1/COM2 receive works as a toggle via the volume knob.

Just the proportional volume adjustment is not implemented yet.

I am building my own simulator hardware, and I have a working audio panel with potentiometers for COM1 and COM2 (and NAV1 and NAV2 that also work in MSFS2020).