rm parent from QcrAction

See merge request !11

protect against empty registry etc

See merge request !10

QcrDoubleSpinBox: correct default arg values to get rid of compiler warning

See merge request !9

CMake: set CMAKE_LIBRARY_OUTPUT_DIRECTORY to /lib

See merge request !8

Qt6 Migration

See merge request !7

Also added sections to build libQCR with Mac CI-machines.

`QString::size()` returns a `qsizetype` which must be converted to an int before
being consumed by `std::max`.

Update catch2

See merge request !6

Prepare for Qt6

See merge request !4