[agb] Added function to change the text color

[tolik518]

Currently you need to create a new writer everytime you want to change the color of the text.
I implemented these 2 functions to make it more straight forward and to recycle the writer instead of creating a new one for each color.

set_foreground_colour and set_background_colour can be used like this:

    gba_write!(&mut writer, "Lorem ipsum dolor sit amet,").unwrap();
    writer.set_foreground_colour(2);
    gba_write!(&mut writer, "consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt.").unwrap();
    writer.set_foreground_colour(3);
    gba_write!(&mut writer, "ut labore et dolore magna aliquyam erat, sed diam voluptua.").unwrap();
    writer.set_foreground_colour(5);
    gba_write!(&mut writer, "At vero eos et accusam et justo duo dolores et ea rebum.").unwrap();
    writer.set_foreground_colour(6).set_background_colour(1);
    gba_write!(&mut writer, "Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.").unwrap();

• Changelog updated

[tolik518]

Color changes should be implemented like in the ObjectTextRenderer instead, like it was mentioned in #699