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

@Daniel Fore: After migrating the replace all to GtkSourceView the UI is still blocked, so adding a spinner and running the replace all in background seems to be a must, however in this context I would also suggest:
* adding a button to cancel the replace all action
* set the search and replace ui insensitive, as you shouldn't be able to run another replace all while a replace all action is in progress
* set the textbuffer not editable, as the text shouldn't be editable

What do you think?