...which, according to callgrind, reduces instruction fetch count spent on
compiling sw/source/core/layout/paintfrm.cxx (randomly selected because it is
rather large) by 5% from 41,992,064,226 to 39,861,989,855 (function main() in
clang-6.0).

This is best done by forwarding ignoreLocation calls from Plugin to the
PluginHandler signleton, but due to the tight mutual coupling between plugin.hxx
and pluginhandler.hxx that unfortunately required some reorganization (and two
outstanding TODO clean-ups of temporarily introduced using declarations in
plugin.hxx).

Change-Id: Ia4270517d194def7db7ed80cb6894e9c473e9499