minor adjustment to the GUI package manager

Avoid using some GUI elements in the wrong thread, just in case.
raco pkg install: make `--force` override scope-conflict errors

It's possible that an installation will have a package X already and
a user wants to install a different X. To make it all work out,
the user may have to also install a new Y for every installation-scoped
Y that refers to X --- maybe not easy, but at least possible as
a last resort.