Remove Qt dependency

[romangg]

Let's define the long-term goal to remove the dependency of Qt from this library.

This is similar to https://gitlab.com/kwinft/wrapland/-/issues/1. Disman uses Qt to a larger degree though with its plugin systems and D-Bus usage. So getting rid of Qt might take longer.

element	plan
CMake	keep
PImpl idiom	keep
Q_PROPERTY	What about KDisplay models?
Qt containers	std containers
QSharedPointer (	std::shared_ptr
QScopedPointer	std::unique_ptr
QPointer	std::unique_ptr (?)
QThread	std::thread
QEnum	What about KDisplay models?
Qt5::Test	same as Wrapland
event system	same as Wrapland
QPoint, QRect,...	same as Wrapland (Mir)

[romangg]

marked this issue as related to #16