Port QtVCP from PyQt5 to QtPy

[bgermann]

Replace all PyQt5-specific imports and idioms with their QtPy equivalents, making the codebase compatible with PyQt5 and PyQt6.

Import changes:

• Replace from PyQt5.XXX import with from qtpy.XXX import
• Replace bare from PyQt5 import XXX with from qtpy import XXX
• Replace pyqtSignal, pyqtSlot, pyqtProperty with the qtpy canonical names Signal, Slot, Property
• Port PyQt5.QtDesigner imports to qtpy.QtDesigner
• Port PyQt5.Qsci imports to qtpy.Qsci

PDFViewer: add QtPdf backend as Qt6 alternative to popplerqt5

• Try qtpy.QtPdf.QPdfDocument first; fall back to popplerqt5
• Use QPdfDocumentRenderOptions with Annotations flag for render quality matching the popplerqt5 Antialiasing/TextAntialiasing hints

sys_notify: make DBus main loop integration backend-agnostic

• Try dbus.mainloop.pyqt5, then pyqt6.

qtplasmac handlers: replace find_spec("PyQt5.QtWebEngineWidgets")

• Use a module-level _WEBENGINE_AVAILABLE flag set by try/import of qtpy.QtWebEngineWidgets in the qtplasmac handler

plugins/status_label_plugin: fix pyqtProperty descriptor check

• Replace fragile 'PyQt5.QtCore.pyqtProperty' in str(...) with isinstance(..., Property) to work across all Qt bindings

[andypugh]

@c-morley The description sounds like a good approach. What do you think?

[snowgoer540]

Made some time to apply this to the project and test it out.

After installing the necessary pyqt dependency, I get these in the terminal window when running the qtplasmac GUI:

QSocketNotifier: Can only be used with threads started with QThread
QSocketNotifier: Can only be used with threads started with QThread
QSocketNotifier: Can only be used with threads started with QThread
QSocketNotifier: Can only be used with threads started with QThread

[bgermann]

QSocketNotifier: Can only be used with threads started with QThread

I have removed the dbus.mainloop.glib fallback (which was added for PySide which we do not need to support) so this should be fixed.

[snowgoer540]

It still gave the same QSocketNotifier message. In order to fix it, I had to change the order of the import test to prefer testing pyqt5 first.

From:
for _mod in ('dbus.mainloop.pyqt6', 'dbus.mainloop.pyqt5'):

To:
for _mod in ('dbus.mainloop.pyqt5', 'dbus.mainloop.pyqt6'):

[bgermann]

It still gave the same QSocketNotifier message. In order to fix it, I had to change the order of the import test to prefer testing pyqt5 first.

Applied to the PR.

[snowgoer540]

From a QtPlasmaC standpoint this seems ok to merge. It will require RIP users to install the python3-qtpy dependency.

Will require @c-morley 's review prior to merging, he is much much more familiar with all of the core qtvcp stuff, and has many more affected GUIs.