Add per-layer water dip control

[oskay]

Currently, layer name is used only to manage which color will be painted. It could also be used to control give per-layer control over water dipping while in Watercolor style painting modes.

For example,
2@-Red
3&-Orange

Where the symbol following the number would be interpreted to control that layer's water usage in watercolor or watercolor+post-dip modes.

Suggested symbols:
Symbol "&" : Force water dip after re-inking brush (that layer only)
Symbol "@": Disallow water dip after re-inking brush (that layer only)

[oskay]

Additional ideas to expand the idea of per-layer control:

Symbol !, as in "2!-red" could force dipping in ink before every individual path (and possibly prohibit prevent re-inking on said paths), which might be useful for dip pens (e.g., quill pens).

[oskay]

Additional idea: Per-layer painting height control.
For example,

3%67-Orange could control the brush height for painting to be at 67% for that layer.

[oskay]

Additional thoughts, as per suggestions here:
http://forum.evilmadscientist.com/discussion/392/watercolorbot-and-inkscape-control-questions-and-feature-enhancement-requests

• A pause (and wait for paint to dry...) option could be added in a similar way:
  Suppose that a layer name started with a number between two #-signs: that number could (within limits) be used as a pause signal. For example, a layer named "[Feature Request] wipe out excess paint drop #30# - wait for orange paint to dry" would pause for 30 s.
• Manual water dip, wash, and re-inking options could be added in a similar way:

D1 - Dip in water 1
W1 - Wash in water 1
P1 - Get paint 1

I'm not sure how useful some of these commands would be; they would lead to documents with a huge number of layers, especially if multiple dips and washes are manually requested. Another method that has been used by other extensions (e.g., G-code extensions) is to read and parse text embedded in a given layer to give parameters. That might work well for us.