Created attachment 182061 [details] [review]
First proposal

Given that undo button is pressed while preediting, we need undo/redo methods for GtkIMContext since each input method only knows about the last operation in the input method.

For example in gedit with Japanese input method,

1. Input "1234" directly
2. Input "asuha" through Japanese input method at the tail of "1234"
3. Then gedit shows 1233[あすは]
  [] means preedit string here.
4. Convert "あすは" to "明日は" (not commit yet at this time) 
5. Press undo button

Actually gedit delete "1234" string and leave [明日は] as it is, but users expect to be revert the conversion. i.e. gedit should show 1234[あすは].