qt5-qtwebkit: Update to 5.212.0-alpha4

WARNING: This release is based on old WebKit revision with known unpatched vulnerabilities. Please use it carefully and avoid visiting untrusted websites and using it for transmission of sensitive data. Please wait for new release from qtwebkit-dev branch to use it with untrusted content.

### New APIs

* `QWebSetting::ErrorPageEnabled` - `true` by default, allows to disable built-in error pages if they are not desirable

### Improvements

* [CSS Compositing](https://www.w3.org/TR/compositing-1) is now supported
* Push is enabled by default for HTTP/2 requests
* QtWebKit does not require Python 2 anymore for building and can use Python 3 instead
* QtWebKit won't be eager to pick bitmap fonts as a suitable choice for standard font families
* More appropriiate hinting option for web fonts is used

### Bug fixes

* Fix memory leak in [CustomEvent](https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/CustomEvent)
* SVG fragment identifier is not respected if it is a part of an HTTP URL  (#818)
* Context menu actions for images in Resources tab of inspector use blob: instead of original URL (#899)
* Always enable JavaScript for QWebInspector (#566)
* Inspector UI: Only selected item is painted in Styles combobox (#262)
* Inspector should inspect itself in DEVELOPER_MODE only (#444)
* fast/gradients/css3-color-stop-invalid.html - box is green instead of white (#230)
* Work around [QTBUG-77308](https://bugreports.qt.io/browse/QTBUG-77308) when using Qt < 5.14
* Fix compilation errors with ICU 65.1 and Qt 5.14
* Fix compilation when QPdf is disabled in Qt
* Fix compilation with MinGW for x86_64 target

### Third-party software updates

Windows binaries are shipped with `ICU 65.1`, `libxml2 2.9.10`, `libxslt 1.1.34`, and `libjpeg-turbo 2.0.3`.

## Sponsors

This release is sponsored by:
* dizmo AG
* Florian Apolloner
* Movable Ink and Michael Nutt
* ScrapingHub

(nia)