power: Low power notifications

This is a partial fix for #686 (closed), which changes the notification text. The main goals are to:

• avoid repetition, such as the identical "battery critically low" headings, and the "shutting down soon" / "about to shutdown"
• remove ambiguity where possible, such as the "shutting down soon"
• where possible, prioritise the battery percentage, since this is what actually determines when the notification is shown
• avoid using the time estimates, since they're often inaccurate

The notifications currently look like:

After the MR they look like:

I'd like to change the icons as well, so they look like this:

That could happen separately from this MR, or with it, but it will require someone with more coding skills than me.