Use override in a few more places to avoid compiler warnings

Clang complains about inconsistent use of virtual and override, and
for some reason the only offender when building against Qt 5 was
multilineedit.h. This commit fixes the ungodly amount of warnings that
that one header produced.

When building against Qt 4 we still get warnings from the various
application classes; however fixing them triggers a million new
warnings related to Q_OBJECT which is beyond our control, so we'll
just have to wait until we can finally deprecate support for Qt 4
for good...