Fix uBlockOrigin/uBlock-issues#341 and other smaller problems

[hawkeye116477]

These are mainly fixes for uBlockOrigin/uBlock-issues#341 (rules like cooking.nytimes.com##.nytc---modal-window---isShown:not(:has(.nytc---largepicturemodal---contentBody > .nytc---x---x.nytc---largepicturemodal---xBtn[role="button"]) should start working properly) and few other smaller problems. All tested on Basilisk and appears that works fine.

[JustOff]

Thank you very much for your efforts, everything looks good and I'm going to merge this PR after testing it in my primary working environment and receiving feedback from several active users.

I would also like to note that so far when porting from upstream I always avoided using ES6 semantics to keep browser compatibility at the same level when the legacy uBlock version was separated. Your commits violate this rule, but I think it's time to just increase the minimum supported version of the target browsers to facilitate further work on the extension and not waste time converting the upstream code to the old js syntax. This will mean that starting with the next version, the minimum requirements will be Firefox, Fennec and Thunderbird 45+, Pale Moon 28+ or SeaMonkey 2.46+.

[temp10min]

With this change maybe it is time to change the name to uBlock-for-firefox-XUL (or whatever is more correct) and save "legacy" for the older browsers like PM27?

[JustOff]

@temp10min
This was already discussed here and the answer is no.

PS: Please don't comment on merged pull requests.