use correct width of non-english character in Button

[python-and-novella]

The test code is:

from prompt_toolkit.shortcuts import message_dialog

message_dialog(
    title='简单对话框',
    text='回车或者点击确认',
    ok_text='确认').run()

It should show Button's right_symbol:

[jonathanslenders]

Thank you @python-and-novella