Errors in the file output templates are reported in mono-spaced font,
so that the arrow character ^ is displayed at the right column
pointing to the offending spot.
A static symbol Slic3r::SEMVER is introduced, which holds
the running slicer's Semver object.
This is mainly done to make testing updater behaviour
_much_ easier. Additionaly to cleanup some questionable code
(Semver was being parsed multiple times / in multiple places
in the frontend.)
