Qt6 migration

[Tofee]

This is a proposed Qt6 migration for qmltermwidget, inspired from lxqt#468 .

Note that I dropped the Qt5 build fallback, as I didn't need it on my side.

[kalaksi]

I can confirm that this works, but the font width is somehow wrong. It even changes during selection. It's useable, though, but not good.

[kalaksi]

I thought the issue might be related to warning I was seeing:
Using a variable-width font in the terminal. This may cause performance degradation and display/alignment errors.

I get that warning even with the default monospace font.
Setting font.setStyleHint(QFont::Monospace); or font.setStyleHint(QFont::TypeWriter); didn't solve it. font.setFamily("courier"); seemed to resolve the warning but not the actual problem.

Eventually, I found that upstream qtermwidget has solved multiple separate problems relating to font width (mostly for non-ASCII). One of those solved this problem for me: c880ab2.

I have it cherry-picked here: https://github.com/kalaksi/qmltermwidget/commits/qt6-font-fix/

[ctrlcctrlv]

This should be merged.