Improve QML iteration speed for shorter build times

With Qt 6, we have more comprehensive QML tooling and build system support than we had with Qt 5:

• We automatically generate qmldirs
• We move files to other places so that tooling can find them in the correct places
• We automatically run qmlcachegen/qmlsc for better runtime performance

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However, we have now received multiple reports about slow compilation speed and frustrating compilation speeds. This epic is about investigating and improving the situation.

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An initial investigation should focus on:

• Which steps take the most time? Where are we actually spending time
• Does the above depend on the used generator?
• Does the above depend on the operating system?
• How much of the work we do can be parallelized? Do we have sequential bottlenecks.

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Once the above has been done, we can fix deficiencies, and add knobs to configure trade-offs, like a setting to only run qmlcachegen in release builds.