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.