Fix issues with buffer selection and filtered views

Whenever the current buffer changed, filtered BufferViews need to
check if the previously selected item should be hidden (e.g. due to
activity settings). The existing code for this never really worked
due to the currentChanged() signal emission being wonky (it would
sometimes be eaten for some reason), and fully stopped working in
Qt5 since it connected to a private, internal slot of QSFPM.

This commit removes the old code and replaces it by faking the
emission of a dataChanged() signal from the base model whenever
the current buffer changes.