widgets, gui: Connect GimpToolBox to icon size changes

Inspired by @pixelmixer and issue 9745.

This patch allows the Toolbox's active image, brush, pattern, and gradient indicators to scale based on icon size preferences. The scale ratio is based on the original hardcoded values and is intended to match the size of the Color Selector area.

Note that if the pattern itself is smaller than the new size, it won't scale up. This is likely due to an issue with the patterns renderer, and will need to be fixed separately.