Disconnect from UI/Engine signals when closing app
Disconnect from UI/Engine signals when closing app

This prevents a lot of extraneous (and often erroring) operations
when Geary closes, particularly due to the conversation and folder
list selections changing as items are removed.

mjog/geary,robol/geary,AtticHacker/geary,glebihan/geary,robol/geary,jimbonator/geary,AtticHacker/geary,mjog/geary,martinra/geary,tshikaboom/geary,githubnemo/geary,tshikaboom/geary,glebihan/geary,robol/geary,martinra/geary,glebihan/geary,AtticHacker/geary,jimbonator/geary,martinra/geary,aatishnn/geary,tshikaboom/geary,githubnemo/geary,jimbonator/geary,mjog/geary,githubnemo/geary,aatishnn/geary,emersion/geary,aatishnn/geary,emersion/geary,emersion/geary