Issue #8866: Text tool needs direction/position setting for the outline

This patch attempts to resolve #8866. It adds a new "Outline Direction" option to the Text Tool with three options:

1. Inner: Current outline behavior
2. Centered: Turns off the clipping on the outline so that the outline border can be seen inside and outside the text path.
3. Outer: Turns off clipping and changes the draw order so that only the outer border can be seen. When the style is set to "Outline only", it also clears the path region so that only the outer border is seen.

Example of the different options:

Text_Outline_Direction_Examples

The outline width is added to the GimpText border attribute calculations in gimp_text_layout_position () so that the layer borders expand and contact as you change the outline (in a similar manner to increasing/decreasing the size).