Thus the first_layer_height is no more object specific. That makes
a lot of sense due to the brim calculation being performed over
all layers at once and due to future merging of supports of
different objects at first layer.
2) Because now first_layer_height is print specific, the relative
first layer height derived from the object layer height was partially
disabled: First the relative first layer height is converted to
an absolute value when importing config, second the side text
was changed from "mm or %" to "mm". Still the UI allows entering %.
Both changes may be controversial, let's wait for user feedback.
1) All slicer's exceptions are now derived from Slic3r::Exception.
2) New exceptions are defined for slicing errors.
3) Exceptions are propagated to the Plater to show.
It remains to modify the slicing back-end to throw the new SlicingError
exceptions instead of std::runtime_error and to show the other exceptions
by a message dialog instead of a notification.
Exceptions thrown by Flow calculation were made explicit classes derived
from std::invalid_argument.
The PresetHints::recommended_thin_wall_thickness() newly catches these
exceptions and it shows the reason of why the hint is invalid.
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.
