qt: use DialogsProvider::getInstance(m_intf) for exit
On KDE Plasma, the QuitSpy is triggered without the DialogsProvider being initialized, which triggers the assertion in DialogsProvider getInstance() method.
vlc: ../../../../modules/gui/qt/dialogs/dialogs_provider.hpp:101:
static DialogsProvider* DialogsProvider::getInstance(): Assertion `instance' failed.
We can provide the p_intf ourselves at initialization, but we need to make sure it's correctly destroyed afterwards. Since it's destroyed afterwards in ThreadCleanup, this will be correctly deallocated.
Fixes #29205 (closed)