Code review comment for lp://qastaging/~evfool/scratch/lp1309291

@Daniel: after experimenting on how to make replace all async, and asking the gedit developers, it turned out that replace all is fast enough (less than 1 second on a 3 mb text doc) with some attention (textbuffer changed signal handling after each replace did slow it down to more than a minute), that is why the gtksourceview devs did not add an async replace all.

So I have tweaked the code a bit, and now replace all seems fast enough for me to avoid the need for spinner, cancel button, and stuff like these. Could you please try and comment if you're satisfied with the result?