Also get rid of various old references to menurc and don't install it anymore to
etc/ (neither the new shortcutsrc as it doesn't look like it brings much value
to do so).
Add a default toolrc file, installed in the sysconf dir, with a
more minimalistic tool layout using groups, inspired by Photoshop
(not identical, though, don't kill me! :)
It must not be a boolean but a `feature` option, with `auto` by default.
`auto` value mean enabled for macOS and Win32, and disabled for other
cases. This default logics disappeared in the meson build.
Also the mypaint-brushes package is a mandatory dependency, which must
always be checked. Absence is fatale.
Finally properly set the MYPAINT_BRUSHES_DIR macro depending on the
proper relocatable case.
Jehan