Bug/Search for Qt6 then Qt5 before building

[lgtmak]

There is a bug with find_package in which the ordering is not respected on Linux:

Linux Bug

This makes the qsk_setup_Qt macro always load Qt5 (as opposed to suggested practices).

This will check for the Qt6 first then fallback to Qt5 if not found, thus replicating the best practice behavior.

To target a specific version pass QSK_QT_VERSION to the cmake build command:

$ cmake -DQSK_QT_VERSION=Qt5 ../ && make

Documentation to be added in #351

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[lgtmak]

Perhaps this is not the best approach and we should instead build the library for both Qt5 and Qt6?

Such as providing build time checks for packages or cmake vars then building targets as QSkinny::QSkinny6 and QSkinny::QSkinny5

[lgtmak]

There is also two CMAKE vars that could be used instead:

CMAKE_FIND_PACKAGE_SORT_ORDER=NAME

CMAKE_FIND_PACKAGE_SORT_DIRECTION=DEC

[lgtmak]

I decided on a hybrid approach, ready for review and merge.