physical_printers.
Updated the snapshots dialog to display sla_print and sla_material.
Updated the snapshots dialog to display physical_printer instead of
printer if the physical_printer key is nonempty.
The "physical_printer" key was moved from [extras] to [presets] section
of PrusaSlicer.ini
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.)
