1686: Add support for generating and uploading checksum files for releases

[kduske]

Closes #1686.

Notes:

• I had to convert the appveyor build script to an external batch file since I needed to call another batch file, and I needed to use labels, both of which I couldn't get working with the embedded powershell script.
• The checksums are created using cmake itself via the md5sum command. To obtain the correct filenames, we generate the appropriate scripts in TrenchBroomApp.cmake by converting cmake/GenerateChecksum.bat.in and cmake/GenerateChecksum.sh.in.
• There is a test release with the results here: https://github.com/kduske/TrenchBroom/releases/tag/v2.0.0-checksum_test_3

[ericwa]

Looks good, my only question is will
echo "Building TrenchBroom failed"
in appveyor.bat be enough to make to make the appveyor build fail?
Does it also need the .bat equivalent of "exit 1"?

[kduske]

Done, thanks! I'll merge it now.