Previously, we were rewriting the on_primary_color_change in the Text
Tool and Gradient, which made the palette widget no longer update after
picking a color from an image. Additionally, it also crashed the program
after leaving the Gradient tool and trying to change color.
This ensures that the gradient does not paint over the rulers at
any zoom level, and also shows the guidelines/handles even when the
gradient is clipped.
This patch adds a new gradient tool to pixelpaint that allows us to fill
the canvas with a smooth color gradient outline at the edge of the
filled area.
