Issue 3406: convert-ly: Flag.transparent and Flag.color inherit from Stem

Issue 3406: convert-ly: Flag.transparent and Flag.color inherit from Stem

After issue 3395, Flag color and transparency does not need to be
changed in synch with Stem color and transparency.

Strictly speaking, it is overkill to change synchronized reverts along
with synchronized overrides: this does not work with things like

\override Stem.color = #red
\override Flag.color = #blue
...
\revert Stem.color
\revert Flag.color

where only the reverts get conflated (and thus Flag color stays in
place).  That's less than perfect, but probably better than leaving
reverts alone (for Flag.transparent, they might have been created
automatically by an earlier convert-ly rule).