Add brush controls

[manthey]

Closes #147.

Closes #141. The current style group and brush settings are stored in local storage.

Currently, circle and ellipse annotations are converted to polygons when they are combined in some manner. This could be done in a coarser manner to reduce the number of vertices. The current setting is no more than 1/10th of a pixel of error at the maximum zoom level (typically this is twice the resolution of the base image, so 1/20th of a base image pixel, though that can be increased). Perhaps something like 1/2 of a base pixel would be sufficient and speed up some of the processes.

Also, this currently becomes slow with a large number of annotation elements because it updates the entire annotation feature. To speed this up, we probably would need to render the annotation as two sets of features (unchanged and recently changed), but that would involve more book-keeping. Once this is in use, we'll see where the limit of performance is located and adjust accordingly.

[manthey]

This needs to add keyboard and control documentation, but that will be done in a subsequent PR. 'b' switches to brush mode. 'B' cycles through brush shapes. 'z' and 'x' change brush size.