bug#25214: #25214 26.0.50; Interacting with user from threads other than the primary

[Zhang Haijun]

Every inputting request from other threads can be treated like an email. Emacs 
puts all requestes in in-box and show a notification icon(or number) on 
mode-line. Then user can open the in-box to process an email(inputting 
request). For example user can run a command like `list-inputting-request` to 
open a buffer to show all inputting requestes. User can choose any request to 
process. If an inputting request is chosen by user, then Emacs switches to the 
thread to process the inputting.