Synchronize some editor functionalities

[jspenguin2017]

I started Nano expecting gorhill to never add an IDE to uBO, but it happened, and it was done pretty well. 3 months ago, Nano was clearly superior when it comes to crafting filter rules, but now uBO is catching up real fast, and we are falling behind. Although we might need to re-think our future, we'll finish pulling recent changes in the meantime.

My filters - Upstream changes rejected for now, I will implement these functionalities:

• (Done) Ctrl + S for saving
• Proper last line break handling (optional)
• Clear undo history on load (optional)

My rules - Upstream changes accepted

Whitelist - Upstream changes accepted, I will (re)implement these functionalities:

• Warn suspicious RegExp rule
• Clean up dead background code

Assets Viewer - Upstream changes rejected

[jspenguin2017]

I'm really busy these weeks, hopefully I can get them done by next week.

[jspenguin2017]

Looks like we'll need to rebase... Anything else should I push to upstream before I do that?...

[jspenguin2017]

Most are sorted out.
The rest are tracked in #1 .