Improve visual style of code folding

[drammock]

Description of your problem

I run spyder from source using bootstrap.py, I'm currently on commit 65bc403. New (to me) is the gutter area separating the line numbers from the user-entered code in the editor pane. I discovered that the intended use is code folding (which I know some people really like), but I find this implementation really unpleasant:

1. it breaks visual connection between number and line
2. every time my mouse rolls over it, the gutter flickers to a darker color
3. if my mouse hovers for slightly longer, portions of the editor window change color to indicate what will not be folded if I clicked.

The high-contrast editor background color changes are especially jarring (changes from my normal black background to a very light grey).

suggested improvements

• if there must be a gutter there, at least make it the same color as the background behind the line numbers.
• instead of highlighting portions of the gutter, implement code folding as little triangles or chevrons on the first line of the part to be folded (like in Sublime or Atom)
• don't change the background color of non-folded parts of the code (neither Sublime nor Atom do this)
  • If you insist on showing users what is going to be folded, include a new key in the color palette called "folding highlight color" or something similar, so that users like me (who don't want this behavior) can set it to the same color as their editor background (effectively turning off the feature)