feat: add KDE Plasma 6 window positioning handler

[SemmerSky]

Problem

The existing kscript handler stopped working in Plasma 5.71 due to API changes (workspace.clientList() and client.geometry were removed). On Plasma 6 Wayland with multiple monitors, NormCap overlay windows pile up on the primary monitor instead of spanning all screens — making it practically unusable in multi-monitor setups.

Solution

New kwin6 handler using the updated Plasma 6 JavaScript API:

• workspace.windowList() (with workspace.windows as a fallback property for minor-version differences)
• window.frameGeometry = Qt.rect(x, y, w, h) for positioning (stable since Plasma 6.0)

The handler is selected automatically on KDE Plasma 6+ and falls back gracefully to kscript on Plasma 5.

Implementation notes

• Uses jeepney for D-Bus calls, consistent with ADR 006
• json.dumps() for the window title in the JS snippet correctly escapes quotes, backslashes, and newlines in window titles
• Unloads any leftover script before loading fresh, fixes repeated captures in edge cases
• Cleans up the temp .js file in a finally block on all code paths
• @functools.cache on _get_plasma_major_version() to avoid repeated plasmashell subprocess calls

Tests

13 tests covering:

• is_compatible() for KDE/non-KDE, Plasma 5/6, unknown version
• is_installed() returning bool without raising
• JS generation and D-Bus call verification
• Special character escaping in window titles (quotes, backslashes, newlines, tabs)
• Graceful handling when KWin rejects the script (negative return code)

Testing

Tested on Fedora 44, KDE Plasma 6.3, Wayland, 3-monitor setup. All 3 monitors are positioned correctly.