text-input: Reset the input method state of previous user input

Both GtkIMContext and IBusInputContext support the reset function.

Add reset request to allow wayland input method module to tell input context to forget the internal state of previous user input in input method, like the '"' symbol.

If input '"' first time, "“" will be inputted; if input '"' again, "”" will be inputted.

When user switch the focus widget, input method module needs to use this request to tell input method to forget the state of previous user input.