fixes on ratio_over
fix Flow::extrusion_width (bad computation of first_layer_height)
fix enum visibility
moving test classes to prusaslicer test directory (wip)
all that because i was trying to write a test class for a modification in min_object_distance (and i didn't even start)
by the PlaceholderParser.
Namely, all the options with the "ratio_over" reference are now handled
correctly by the PlaceholderParser with the exception
of the "first_layer_extrusion_width", which overrides speed of extrusions
by their respective extrusion type.
Also the various extrusion widths (extrusion_width, first_layer_extrusion_width,
external_perimeter_extrusion_width etc.) produce the same numbers
as if ran through the back-end, with the assumption of not overriding
layer height by the variable layer height editing tool or layer height
modifiers.