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.