QEvdev: Replace manual memory management with unique_ptr

Make create() return, and m_mice/m_keyboards/etc store, handlers by unique_ptr.
In most cases, we can't use qt_make_unique(), since the ctor we're calling is
marked as private.

Since QHash can't hold move-only types, use a std::vector<{QString, unique_ptr}>
instead. As this pattern repeats in all four QEvdev*Manager classes, create a
small class template.

Saves almost 6KiB on optimized Linux AMD64 GCC 9.1 builds across all .so's that
link to QtInputSupport.a.

Change-Id: I8f62b6b629d6e1855314c0a4fb4fc069db9ae0ce
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>