Design
Currently a viewport can be toggled into cameramode which shows either the scene's active camera or the viewport's local camera (when use_local_camera is enabled). This means it's not possible to directly view a camera without also activating it or setting it as the local camera first.
This can be improved by allowing to view any camera which is available in the viewport. A selector to choose which camera to show could replace the viewport name that is already displayed and make it interactive instead. This would make it something like a enum menu of what the viewport can display which could also present different view options like the viewpoints or persp/otho toggle.
Implementation
This patch allows the view3d.view_camera operator to switch to any camera by accepting a camera_id string. The operator won't affect the scene's active camera but rather store the camera local per viewport with the already existing view3d.camera property. When leaving the cameraview that property gets synchronized back with the scene camera.
As the viewport then always uses a local camera the use_local_camera option becomes obsolete.
