We'd like subclasses of GimpPaintTool to be able to issue paint
commands to the tool's paint-core (in particular, see the next
commit.)  Since paint commands should be executed on the paint
thread, the subclasses must not call the paint-core functions
directly, but should rather let gimppainttool-paint issue the
commands on their behalf.

Reorgainze gimppainttool-paint to make it usable for this purpose
by subclasses.  In particular, add
gimp_paint_tool_paint_core_paint() and
gimp_paint_tool_paint_core_interpolate(), which call the
corresponding paint-core functions on the paint thread.

Additionally, rename the {start,end,flush}_paint() virtual
functions of GimpPaintTool to paint_{start,end,flush}(), and rename
gimp_paint_tool_is_painting() to gimp_paint_tool_paint_is_active(),
so that all the gimppainttool-paint-related stuff are grouped under
the same namespace.