Add Kapsule container detector

Lasath Fernandorequested to merge
work/fernando/kapsule into master

Add KapsuleDetector for detecting and listing Kapsule (Incus-based) containers. Uses libkapsule-qt's KapsuleClient to list containers via D-Bus and relies on OSC 777 escape sequences for in-session detection (same pattern as ToolboxDetector).

The feature is gated behind -DWITH_KAPSULE=ON and requires the Kapsule and QCoro6 packages to be installed. When disabled, Konsole builds without any kapsule dependency.

Some more info about Kapsule:
https://blog.lasath.org/2026/02/kapsule-completing-kde-linux.html
https://blog.lasath.org/2026/02/kapsule-it-shipped-and-nobody-died.html

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