Draft: Issue #7581: Let user choose guide color

This patch attempts to resolve issue #7581 (and possibly sets the stage for 989).

It adds a new color property to GimpGuide so that users can define the main color. Currently only the script-fu dialogues from the main menu provide this option. If NULL is passed in, then the current default color is used. This allows non-GUI ways to create guides without requiring a new dialogue (thus disrupting workflows).

Note: Currently guides go back to the default color if saved as an .xcf file and reloaded. I think there'd need to be a new XCF version to allow colors to be saved and loaded - I'm not sure how that's handled (e.g. if several XCF changes are bundled together per version).