Display margin glyphs on unused declarations
Adjust looks and feels of margin glyphs

It turns out these glyphs can only appear on the left of the text view.
We need a more flexible mechanism.
Use a margin canvas instead of a margin glyph
Use absolute axis for margins
Add click events to margin markers
Place the markers on top of vertical scroll bars
Move all constants to a separate module
Navigate to exact location of unused declarations
Markers appear right after colorization
tune unused declaration marker visual representation
Merge pull request #6 from vasily-kirichenko/unused-decl-markers

tune unused declaration marker visual representation
Add tooltips and documentation
Handle view height changes and fix marker sizes
Merge pull request #601 from dungpa/unused-decl-markers

Initial version of unused declaration markers