[tagwindow] work around slow mnemonics in gtk3
[tagwindow] work around slow mnemonics in gtk3

I can't find out if this is configurable, so I have to assume it's not.
Now I do it "manually". This doesn't happen for the mnemonics on the
stock buttons, so they are still slow, but I don't actually use those,
so I don't care.

drougge/wellpapp-pyclient