Windows: Add Qt6 and Editor install script

[Ghabry]

The buildscripts part to get the editor (soon) running again on Windows.

In short running build_editor.cmd and download_qt6.cmd gets everything setup.

The PR to the editor repo follows later. This needs a bit more work.

[Ghabry]

~~uhm have to shorten the triplet name a bit. I wonder since 2 days why CMake while buliding Kirigami reports "unknown error". Turns out I hit the 260 char path length limit :facepalm: so have to tune everything to make it shorter. hmpf~~

Correction: Enabling Long path support also appears to work.

Run once as administrator in powershell

New-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Control\FileSystem" -Name "LongPathsEnabled" -Value 1 -PropertyType DWORD -Force

Have to document this.