Add peak value display to the level meters

Update documentation for Scarlett 4th Gen and Vocaster

Update documentation for Scarlett 4th Gen and Vocaster

Update documentation for Scarlett 4th Gen and Vocaster

Update documentation for Scarlett 4th Gen and Vocaster

src/Makefile: Respect CFLAGS from environment

Check Firmware Version before enabling Levels menu item

The level meters don't work correctly on earlier kernel versions, so
they shouldn't be displayed in that case. The "Firmware Version" ALSA
control was added to the kernel at the same time as the level meters
were fixed, so use the presence of that to determine whether or not to
enable the menu item.

Update Makefile to get version from $APP_VERSION

If git describe fails, check $APP_VERSION, otherwise set the version
to "Unknown".

Github action to build debian package on release

Add top-level Makefile and RPM spec file for packaging

Top-level Makefile can create tar and RPM files to release.
Also add *.tar.gz to .gitignore.