Improve UI for full styling support

[banesullivan]

Now that #10 is implemented and full styling is supported following large-image's specification, we should provide a more feature rich UI in the templates for making composite images of multiple bands and/or frames.

A common style dictionary might look like:

style = {"bands": [
  {"band": 1, "palette": ["#000", "#0f0"]},
  {"band": 2, "palette": ["#000", "#f00"]},
  {"band": 3, "palette": ["#000", "#00f"]}
]}

I'm imagining a UI that has a tree structure of frames and bands with checkboxes on the bands. Further, you'll be able to select an individual band and change each of the fields available for a band spec. This might look like:

[ ] Frame 1 - name
  [ ] Band 1 - name
  [ ] Band 2 - name
  ...
  [ ] Band N - name
[ ] Frame 2
  ...
  Band N - name
...

Then selecting a Band creates a dialog where you can control the following parameters at least:

• min/max: float fields
• palette: dropdown menu (for now... custom palettes may come later)
• scheme: dropdown choice of linear or discrete
• nodata: float field
• composite: dropdown choice of lighten or multiply