Fix opening new tabs on macos(#518)

[BPerlakiH]

Fixes #518: Open link "in a new window" on macOS.
Partially based on #526

Known issues:

• the opened tabs are not restoring their content on app restart, the tabID remains nil (might need another issue/ticket).
• command+click on a link would not open it in a new tab (as I would expect it, since native safari has this capability)

[kelson42]

@BPerlakiH Thank you.
@automactic Could you please have a look?

[kelson42]

@BPerlakiH Please fix the two weaknesses you have listed so the PR is feature completed. Once done, please update PR comment and request new review.

[kelson42]

@BPerlakiH Rebasing to benefit of the new CI

[automactic]

@BPerlakiH Oops, I am so sorry I did not see you have this PR already opened for the external URL alert issue. So I made a PR (#539) and merged it, because I created the issue in the first place. Apologies for the merge conflict

As I have communicated with @kelson42, I won't have any capacity for PR reviews in the foreseeable future, so I'll give this one a quick look, and let him decide what to do

[kelson42]

@automactic Thanks for having a quick look
@BPerlakiH Please fix the problem reported by @automactic and then we should be OK.

[kelson42]

@tvision251 I would like to have your review on this please.

[kelson42]

I can not judge the can and for the moment can not test it on my own. But if the points revealed by @automactic are fixed, then should be OK.