keg: add function to detect if .app(s) installed.
caveats: print .app caveats if installed to keg.
avidemux: remove duplicated .app caveats.
cppcheck: remove duplicated .app caveats.
djview4: remove duplicated .app caveats.
emacs: remove duplicated .app caveats.
fontforge: remove duplicated .app caveats.
frescobaldi: remove duplicated .app caveats.
graphviz: remove duplicated .app caveats.
jstalk: remove duplicated .app caveats.
macvim: remove duplicated .app caveats.
python: remove duplicated .app caveats.
python3: remove duplicated .app caveats.
qcachegrind: remove duplicated .app caveats.
valkyrie: remove duplicated .app caveats.

Closes #26033.