Issue #8461: Fill/Stroke Path allows foreground/background color as options

This patch attempts to split the "Solid Colors" option (which solely used Foreground Color) for "Fill/Stroke Path" with options for either Foreground or Background Color. It replaces GIMP_FILL_STYLE_SOLID with GIMP_FILL_STYLE_FG_COLOR and GIMP_FILL_STYLE_BG_COLOR.

GIMP_CONTEXT_PROP_MASK_BACKGROUND is added to the Fill and Stroke options so they can be aware of the background color. Any place that used GIMP_FILL_STYLE_SOLID as a default value now uses GIMP_FILL_STYLE_FG_COLOR; since the original behavior was to use the foreground color for "Solid Colors", this should be consistent with user expectations.