Add per-layer control of certain settings.

[oskay]

In a similar vein to evil-mad/wcb-ink#24, it would be nice to have some per-layer control features for the AxiDraw.

The main ideas listed there are chiefly relevant to paint -- water dipping, and re-inking (per stroke and so forth). AxiDraw does not need these. However, it could use some other potential per-layer adjustments, for example pen-down speed and pen-down height.

Current layer-numbering syntax requires only that the layer name begins with an integer in the range of 0 to 100. We could extend this by detecting that layer-name integer, followed by a specific escape sequence, followed by another integer, perhaps in the range of 0 to 100. (Certain integers may be excluded, depending on the escape sequence.)

The escape sequences could be +H for pen-down height and +S for pen-down speed, not case sensitive. When printing a layer with one or more layers that contain parameters like this, the specified pen-down height or pen-down speed would override the defaults given in the extension, for that layer only. Since the speed cannot go to zero, the integer following +S would need to be in the range 1 to 100. If more than one valid parameter of a given type is found (say, more than one valid speed command for a given layer), only the last one detected would have an effect. Escape sequences need to be given at the start of a layer or immediately following the layer number.

Example layer names:
2 forest floor: The layer number is 2, and it is labeled as "forest floor". It prints with the default pen height and speed.

45+S50_forest floor: The layer number is 45, and it is labeled as "_forest floor". It prints with the default pen height, but with pen-down speed of 50, regardless of the value specified in the extension.

100+H5055forest floor: An invalid declaration, since the requested height is 5055 (which is outside of the range 0 to 100). The layer number is 100. It prints with the default pen height and speed, since there is not a valid override.

0+s1+h100 Forest floor: The layer number is 0. It prints with the pen-down height of 100, and with pen-down speed of 1, regardless of the value specified in the extension.

22+S0+H0 Forest floor: The layer number is 22. It prints with the pen-down height of 0, and with the default pen-down speed 1, since the speed value of 0 is not valid.

99+S25_+H50 Forest floor: The layer number is 99. It prints with the default pen-down height, but with a pen-down speed of 25. The height parameter is not valid, since it does not directly follow the other parameter but is interrupted by an underscore. The layer name is "_+H50 Forest floor".

+H50 Forest floor: No layer number given. It prints with the default pen-down speed, but with a pen-down height of 50. (Layer numbers are not required.)

[oskay]

Feature implemented in repository; available for testing.