Adds navigation functionality, listeners in FRB

[munkm]

Has the following features:

• new trait for FRB called canvas_edges that defines the x and y low and high values for the frb
• on changes in this trait, function called buffer_changed in fixed_res_buffer.js is triggered. This creates a new frb object and deposits it into the varmesh
• The FRBViewer class in image_canvas.py now has a function called setup_controls() that returns an accordion with colormap and navigation widgets linked to the colormap and navigation-related traits in the FRB. These include:
  • zoom option
  • navigation buttons for up, down, left and right
  • colormap name
  • min and max values for the colormap
  • is_log value for colormap scaling

Resolves #8