Release candidate promoted to Stable

Finally got around to make 1.2.x the "stable" release, with 1.2.01 being the first version in this series.

For those using the "unstable" branch, this means nothing, but for the rest this replaces what was in 1.0.x.

Among the changes from 1.0:

• there are translations in Czech, French, and German
• "shell integration" allows right-clicking on a folder in a file explorer and have that folder open in MP3 Diags
• you can use "external tools" to open a file or more with some external program from within MP3 Diags
• a CLI mode is now available, where you can see notes and apply transformations to arbitrary groups of files and folders
• some bug fixes

While 1.2.01 was supposed to be a direct copy of 1.2.00, I had to get rid of "unstable" in naming, and hopefully that didn't mess up anything. I also included the latest changes to the documentation (see here) and a few changes to make the program compilable with clang.