Use Toast.closed signal instead of Timeout for undo actions

[leolost2605]

Once elementary/granite#677 gets merged and eventually released it would be good to use the closed signal instead of a timeout to do the irreversible action. Otherwise we could end up with a situation where the user hovered over the toast too long and then clicks undo and nothing happens.

[robog-two]

Omg! This actually happened to me before in Geary and it was soul-crushing! This is definitely important especially because trashing an email is permanent, there's no trash folder or anything like that. (Though maybe there should be? And it follows the same settings from Housekeeping for trashed files?)