feat: check preedit before send message

if there is preedit , that means it is used by input method, then the action of send message will not be passed

before, when the there is still preedit (this is text preview of input method) in sourceview, this time when pressed enter, this key should be handled by input method, because preedit is still exist, but fractal handle it, and do the send message action, this create a lot of problem. Then I think we should check if preedit is exist when pass the send message action , input method can work as it works in telegram, element and etc

here I add a Cell<bool> value to record if preedit is exist in sourceview, if there is preedit, that means input method is still working, then not pass the action , then input method will handle the enter key , not fractal handle it, this can make fractal handle input method better