common-vars.pri: Set PROJECT_VERSION to 0.17

At some point, when still on launchpad(?), releases were spun as 0.17* versions, but PROJECT_VERSION actually was never bumped from 0.15. This leads to a mix of how downstreams are versioning snaphots, sans proper upstream releases, which is needlessly confusing.

In Gentoo, but also at least Fedora and Void Linux, we tend to look at the build system as shipped as authoritative source for what we are dealing with.

See also: https://repology.org/project/signon-ui/versions

Signed-off-by: Andreas Sturmlechner asturm@gentoo.org