Qt 6 Building Porting

[Chiitoo]

Builds and runs (no crashes so far!) with spell checking disabled, on Gentoo Linux with Qt 6.7.2.

Bunches to do still:

• Clean up CMakeLists once decided if Qt 5 will be supported via the same codebase.
• Some bits have been commented out, requiring more figuring out.

[Emdek]

@Chiitoo, these changes for the QtWebEngine seem to be the biggest, but not as big as expected. The most annoying thing seem to be that codec change for the streams.

[Chiitoo]

Did a quick'ish rebase so that this builds again with Qt 6 (with no spellcheck), and some additional tweaks such as 'ifdef' additions but... mistakingly aborted the rebase with them so those did not get in this time around.