Replace BufferChangedCallbacks to BufferObservers in frontend code (issue #427)

Sorry, in my previous PR (#447) i am looked bufferChangedCallbacks only in backend folder, frontend code was missed.
Fixed in this PR.

Right now buffer.AddCallback called only in /backend/textmate/testdata/main.go
I have no idea how this testdata used. Should we change this call to BufferObservers too?