Use black as default color for an inset button label text

Use black as default color for an inset button label text

It turns out that LyX was using red as default color for the
inset button text since ever. However, due to the use of "inherit",
the default button text color was black in practice. This was so
until [897ee2ed/lyxgit], which made the label font not inherited
by default anymore. Hence, all insets that didn't specify a specific
label button color started using red as default color.

This commit restores the previous behavior and introduces the
possibility of modifying this default color.

Fixes bug #12771