Problem: Since Plasma 5.27, KWin's Custom Tiling feature (Meta+T tile editor) hardcodes the Shift modifier for drag-to-tile functionality. When dragging a window with Shift pressed, it automatically snaps to the nearest custom tile. There is no way to disable this behaviour or rebind the modifier key.

This creates conflicts with third-party tiling extensions that also want to use Shift+drag for their own tiling behaviour. Users are forced to either:
1. Use a different modifier for third-party extensions, which is less intuitive
2. Compile a custom KWin build to modify the hardcoded behaviour

Rationale: Prior to integration into KWin core, tiling was more modular and configurable. KDE's philosophy has always been user choice and configurability. Hardcoding a single modifier key without any config option to disable or rebind it goes against this principle.

Requested Feature: Add a configuration option (either in GUI or kwinrc) to:
- Disable Shift+drag tiling entirely (allowing third-party scripts full control)
- AND/OR rebind the modifier key (Shift/Alt/Ctrl/Meta)

This would allow users to:
- Use third-party tiling extensions without conflicts
- Choose their preferred modifier key based on their workflow
- Maintain the flexibility KDE is known for

Use Case: Users who prefer third-party tiling solutions (KZones, Bismuth, Polonium) or have muscle memory from other tiling window managers often want Shift+drag for their chosen solution, not the built-in tiling.

Technical Note: The Shift modifier check appears to be hardcoded in KWin's C++ source (introduced in MR !2560). Adding a config option would be straightforward and maintain backward compatibility (default: enabled with Shift modifier).

STEPS TO REPRODUCE
1. Edit `~/.config/kwinrc` and add the flag `enabled=false` under the `[Tiling]` section.
2. Save config + reload KWin

OBSERVED RESULT
KWin Custom Tiling still enabled. Shift + drag window uses KWin tiling engine

EXPECTED RESULT
KWin Custom Tiling should be disabled. Shift + drag window is now free to be reused by another tiling engine.

SOFTWARE/OS VERSIONS
Operating System: Nobara Linux 43
KDE Plasma Version: 6.5.3
KDE Frameworks Version: 6.20.0
Qt Version: 6.10.0
Graphics Platform: Wayland