Snapshot session on focus, key press and output

Before 3d6c839b, sessions were snapshotted every 2 seconds, and also half a second after the associated view received focus or a key press. This second timer could theoretically be postponed indefinitely.

Commit 3d6c839b tries to save energy by not taking a snapshot every 2 seconds, and instead has a timer that fires half a second after any event received by the application.

This commit goes back to the old behavior, but still without the 2 second timer, and also sets the half second timer after receiving the Emulation::outputChanged signal, which is sent at a maximum 40ms after the emulation receives data. It also copies the idea from commit 3d6c839b of not restarting the timer if it's already started, since otherwise, continuous output could postpone the timer indefinitely.