Add "keyframe switching"!

[strasdat]

At the moment, we only add new keyframes, but there is no mechanism to switch to an old keyframe.

Note that opposed to small scale AR software, we can not simply switch to a keyframe which is close to the current frame, but we also have to make sure that both frames share a significant number of 3d points!

(Maybe because of this difficulty, it might be wise to actually implement a keyframe merger in the back-end instead of keyframe switching in the front-end.)