Add configurable hot-corner actions

[wangchun]

Add action-based hot corners in config. Keep backward compatibility for bare corner syntax. Wire runtime dispatch so entering a hot corner executes the configured action.

Example configuration:

hot-corners {
    top-left
    top-right { power-off-monitors; }
    bottom-right { spawn "dms" "ipc" "call" "lock" "lock"; }
}