support colorbar customization

[kswang1029]

Need new functions for the Image class

including

• visibility (show / hide)
• orientation (right / top / bottom)
• label (no label, a custom string)
• ticks density
• colors of border, ticks, tick values, and label
  etc...

note that in the settings dialog, we can customizing the colorbar a LOT.

[confluence]

This can already be done with various overlay functions on the session, but we can add a function grouping the colorbar options together. This should be on the session object; the same overlay settings are used for all images.

[I-Chenn]

@confluence , @kswang1029:
If we want to configure all the items in colorbar within a single function, there will be 26 parameters in that function, which is a bit bloated. May I group them into 5 functions, say: colorbar_visibility, colorbar_label, colorbar_number, colorbar_ticks and colorbar_border, just like the grouping at frontend GUI?