Qt GUI: Fix build with newer/custom Qt builds and minor improvement

[cjee21]

It was originally written as QFontDatabase::families() since QFontDatabase::QFontDatabase() is deprecated since 6.0 and Qt docs says: "We strongly advise against using it in new code. Call the class methods as static functions instead."

It was changed to QFontDatabase().families() by g-maxime in a8ce718.

Now it cannot be built if using a version of Qt with disable-deprecated-up-to = 0x060000:

error C2280: 'QFontDatabase::QFontDatabase(void)': attempting to reference a deleted function
note: see declaration of 'QFontDatabase::QFontDatabase'
note: 'QFontDatabase::QFontDatabase(void)': function was explicitly deleted

So use a version check and switch between both to ensure it can be built on all versions of Qt.

Also make use of translator->load return value instead of Q_UNUSED. I should have done so from the beginning.

[cjee21]

@JeromeMartinez The install Qt action is failing too often so I pin it to specific publicly released version and also enable cache. In the future maybe can use MediaArea hosted Qt version if going to use a self-built static Qt.