Don't silently ignore creation of new windows

[dpogue]

Platforms affected

iOS

Motivation and Context

Currently, attempts to create new windows (via links with target="_blank" or JS window.open) for pages within the application have no effect and are silently ignored. (Links to pages outside the application trigger the allow-intent functionality and open in Safari.)

Description

With this PR, the default behaviour will be to open those links in the same Web View (without creating a new window).

This also introduces a preference (AllowNewWindows) that allows the creation of a new UIViewController to present the new page in a new Web View frame on top of the existing Web View.
Note: This new window provides no controls, so it can only be closed with window.close() in JS.

Testing

Manual testing in a new Cordova app.

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

Merging #885 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #885   +/-   ##
=======================================
  Coverage   74.40%   74.40%           
=======================================
  Files          13       13           
  Lines        1676     1676           
=======================================
  Hits         1247     1247           
  Misses        429      429           

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[erisu]

Changes LGTM.
I also tested

• building
• ensure the default behavior opens link
• AllowNewWindows set to true

   <a href="test.html target="_blank">

  and

  window.open('test.html', '_blank');

  opens a new window and window.close will close the new window.

[vbraun]

What about iframes with target="_blank" links? These now either replace the app, or open another window on top without any way for the app to close it.

E.g. embedded google maps has a target="_blank" link to open the full screen map. We want to embed it into our app as iframe, so <allow-navigation href="https://*.google.com/*" /> must be set or iOS displays nothing. Then user clicks on the link in the iframe. Used to just not do anything (bad), now just replaces the app (worse).