add support for building qml6 plugin

commit 9f603289

gst-plugins-good: add support for building Qt6 plugin for macOS

commit c963510d

meson: add fix for missing Qt include path

commit 60de85e0

filesprovider: provide method to update categories

Caching the list of categories at __init__ time will miss any categories
added during a recipe's prepare() method.  Provide a way to update the
categories list manually and use that when adding a new category when
enabling a plugin with a separate category.

commit 0c0d00a1

good: add support for building the Qt6 qml plugin

commit 66ace099

meson: update to 0.64.0rc1

commit 2dfdf335

only detect qt5 qmake in detect_qt5()