Replace tab characters by 8 spaces

Replace tab characters by 8 spaces

Our layout engine can't properly handle tabs (and it's doubtful how they should be handled
anyway on random pastes), and it breaks while trying. So now we replace tabs by 8 spaces.

Note that Konversation (and probably some others) currently sends a \t for the "italics" format.
This is non-standard and not very consistent across clients, so I chose to not have a tab trigger italics.
So for pastes, we show something mostly correct, while italic text sent from Konversation will cause
some whitespace instead. I think that's a better tradeoff than the other way round (showing italics
when tabs are pasted).