Replace the custom version-string generation by ARPA2CM's
When using ARPA2CM's GET_PROJECT_GIT_VERSION macro, a header file is always written (even if there's no .git) with a suitable representation of the version number (obtained from PROJECT_VERSION if there is no git information).
The string looks like #define tlspool_gui_VERSION_STRING "v0.0.5-19-g259082b5" (There are also split-apart version components, but those are missing the tweak values "-19-g..."; they can be useful for numerical representations).
The generated static QString was being included in multiple places, so replacing that with a const char[] isn't a big deal (until we want to support translation of the About Box, at least).