network-manager-applet: re-do schema generation

fixes #1355. partial revert of f4d4e12c (introduced by #1313).

meta.priority is set to 6 (lower priority than standard) to resolve the
conflict of an exported gschemas.compiled for evince as for nm-applet.
this linked file is NOT consulted, as both binaries are wrapped to look
for their schemas in their nix store path first.

a better fix would avoid already avoid linking those gschemas.compiled
files into the environment.