app: reorganize gimppaintool-paint
Reorganize/clean up gimppainttool-paint. In particular, move all paint-core interaction during painting to gimppainttool-paint.c, so that we can have more control over what's going on; specifically, enter the drawable into paint mode *before* starting the paint core, so that it picks up the correct buffer. This fixes painting with the paint thread using GimpApplicator, and enables us to use the paint thread with GimpMybrushTool.
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