Add signal GimpTextBuffer::color-applied which is emitted when text is
inserted or when color is applied to a span of text.
In GimpTextTool, connect to the signal and update the global color
history.
Unrelated: rename gimp_text_tag_get_color() to get_fg_color() and add
boolean return values to get_fg_color() and get_fg_color() which
indicates if a color is set on the tag at all. This ended up unneeded
in the fix but is an improvement regardless.
There is nothing in our UI to set background color to text, but this
can be used internally, for instance to display input method's preedit
texts with specific colors.
and also #define the names of the properties we (ab)use, so we don't
have to use magic strings all over the place, and we don't have to use
g_object_get() either.
Michael Natterer