versioning: use app/jamicore git SHAs as the version

Currently, the user-facing version number in the about dialog, is a
build-time timestamp that does help us isolate the project versions
accurately. Ultimately, we need to be able to reproduce issues
present in specific versions of Jami. This commit introduces a new
version number in the form: <client_sha>.<jamicore_sha>, allowing
us to reproduce builds more accurately.

Gitlab: #1820
Change-Id: Ie7e20b20da65284e33e745996c410f539b65080c