Refs #8741. Do a few minor fixes.

Found by Eclipse: Qt3 archaisms and catching exception by value.
Refs #8741. Remove all the trailing spaces.
Refs #8741. Replace currentIndexChanged with activated.

currentIndexChanged is called no matter how the index was changed -
by user or e.g. when the combo-box was cleared. activated is emitted
only when user clicks on the thing, which is the only case when we
want to update the plot.
Refs #8741. Add explicit call to plot update.

Before that, changing the current item would cause the plot update
just because the list of functions was updated. This is not working
with new approach and is really not apparent anyway.
Refs #8741. Remove blockSignals() fix.

It was done to fix the problem with currentIndexChanged() which is
not a problem anymore.
Refs #8741. Split the notion of change and auto-update.

There are still certain things we want to update when the index is
changed to synchronize stuff. So we still need something to
be connected to currentIndexChanged(int) signal, though it shouldn't
cause the plot to be updated.
Refs #8741. Remove exception throwing.

This shouldn't be actually thrown as empty groups selector is an
expected situation - you can e.g. just clear the grouping table.
Refs #8741. Revert "Remove all the trailing spaces."

This introduced a lot of pain when merging.
Merge branch 'bugfix/8741_muon_instr_change_exception' into develop