Improve notification behavior when battery drains while plugged in

Because of this dreaded kernel bug I keep missing the battery running out because it thinks it's plugged in …

When battery drains, show a notification even when AC is plugged in

Only shows the notification if percentage crosses warning/critical threshold, not on startup, to avoid showing every time on login in case of e.g. broken battery.

So that @aacid doesn't hit me in the face with his broken laptop

Update battery notifications if they remain open

They are critical and persistent nowadays, so make sure the percentage is updated live to match the current percentage if it is left open for a prolonged period of time.

Add wording for when battery gets low while it is charged

Don't tell the user to plug in the cable when it's already plugged in.

Suggestions on wording welcome, @ngraham ?