Add support for gradle auto-creating missing changelogversion

[marcardar]

It would be really useful if, when building the app, the changelog.xml could be checked to see if it already contains the changelogversion for the version being built. If not, create a stub with the versionName and changeDate auto-filled.

I know this wouldn't be perfect, because you would essentially need to run the build twice. Another problem is that if, like me, you use "dev" of "b1" etc suffixes in the versionName then you probably don't want a changelogversion for each of those.

Anyway, just a thought!