Implement asynchronous waiting for kakoune session initialization

[falbru]

The kakoune clients have to wait for the kakoune session to be initialized before they can connect to it. The current solution to this problem is for the kakoune session to write a signal to a FIFO file when it has finished initializing, and for kakoune-qt to read and wait for that signal from the FIFO.

However, it would nice for this solution to be asynchronous, allowing other Qt widgets to initialize concurrently while waiting for the Kakoune session. One approach would be to add a signal to the KakouneSession class for when the session has finished initializing, and use a slot for that in the KakouneWidget class.