app: bucket fill tool with a "paint-style" interaction.
Rather than just having a click interaction, let's allow to "paint" with the bucket fill. This is very useful for the new "line art" colorization since it tends to over-segment the drawing. Therefore being able to stroke through the canvas (rather than click, up, move, click, etc.) makes the process much simpler. This is also faster since we don't have to recompute the line art while a filling is in-progress. Note that this new behavior is not only for the line art mode, but also any other fill criterion, for which it can also be useful. Last change of behavior as a side effect: it is possible to cancel the tool changes the usual GIMP way (for instance by right clicking when releasing the mouse button).
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