Updated to account for RestrictContext, option to select active camera, and new operator.

Blender 2.69 complains about nonexistent `scene` attribute in `_RestrictContext` and `objects` in `_RestrictData`, so references to both are removed. Camera list is then created on the fly, rendering `ListCameras` and `ReloadCameraList` redundant, also removed.

It'd be nice if active camera is also selected immediately, so layout artist can immediately tweak its settings and position. Also for other cameras to be hidden, to minimize viewport clutter. I'm not sure if such behavior should be default, though, so it's only triggered if the button is Ctrl-clicked.

Also added a new operator to add timeline marker bound to camera, with simple heuristics to avoid redundant / overlapping markers (won't create new marker, if the last marker up to current frame is bound to chosen camera).