Make Morph Browser uses QWE 6.x (was: upgrade QWE 5.15.15 doen't seem enough for using github in UT smartphone)

End goal of this issue is to make Morph Browser in Ubuntu Touch uses QtWebEngine 6.x series, which will allow us to have newer Chromium bases. This in turn will allow Morph Browser to be more useful, as QWE 5.15.19, being based on Chromium 87, is now very much inadequate for modern websites today.

This will in turn requires many app-facing components to support Qt 6, such as lomiri-ui-toolkit, lomiri-ui-extras etc. @gberh is already looking into that.

Original description followed.

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Noticed since 3 days on my Volla UT (probably other smartphones) a slight display incompatibility (github page, using morph-browser or webber web-app).

In fact, writing bug reports with UT smartphone on github has suddenly become unusable as i tested due to the superposition of unrolled option strips.