Add alpha transparency to SVG backend (issue 330300043 by address@hidden)

[beauleetienne0]

Reviewers: ,

Message:
Adds more flexibility to the coloring of elements in SVG files. It is
disappointing how there is no support in PS, and thus our generation of
PNG. This is useful though in my view, and at least it's available
somewhere.

Description:
Update documentation for alpha channel

This change adds documentation about the new alpha channel parameter
when using `rgb-color`. It also renames the sections and the uses of the
function, while keeping "RGB" in the index in addition to "RGBA".

Add alpha transparency to SVG backend

Adding the alpha channel to the color commands allows for objects to be
of different opacities, leaving more flexibility in coloring. This is
only available with the SVG backend as PostScript has poor support. The
`rgb-color` command has been renamed to fit the alpha channel as
`rgba-color`, with a convert-ly rule associated. By default, the channel
is set at 100% opacity. The built-in colors have also been updated.

In addition, setcolor now outputs values from the 0-255 int range, and
not a percentage in the SVG backend.

Please review this at https://codereview.appspot.com/330300043/

Affected files (+730, -724 lines):
  M Documentation/changes.tely
  M Documentation/learning/tweaks.itely
  M Documentation/notation/editorial.itely
  M lily/axis-group-interface.cc
  M lily/include/stencil.hh
  M lily/lyric-hyphen.cc
  M lily/paper-column.cc
  M lily/separation-item.cc
  M lily/stencil.cc
  M lily/stencil-interpret.cc
  M lily/stencil-scheme.cc
  M lily/system.cc
  M lily/tie.cc
  M python/convertrules.py
  M python/musicexp.py
  M scm/output-lib.scm
  M scm/output-ps.scm
  M scm/output-svg.scm
  M scm/x11-color.scm
  M scripts/musicxml2ly.py