refactor dartdoc panel location and behavior

[devoncarew]

From user feedback, the current behavior of the dartdoc panel can interfere with editing the snippet:

• on a click from a user on a symbol, the panel appears below the editing area; this re-sizes the editor and text locations can appear to jump for users
• clicking on blank space will hide the dartdoc panel; this also resizes the editor

Here are some possible refactorings of the UI:

have the dartdoc panel appear in a fixed location, floating over the editing area

• this might dock on the bottom of the editor, indented ~20 px left, bottom, and right
• the editor would not resize
• the dartdoc panel might obscure some content

have the dartdoc panel only show when double clicking a symbol

• there would be less unintentional invocations of the dartdoc panel
• there would be less discovery of how to show docs

have a 'toggle doc' button on the bottom status bar

• a user would hit the toggle to show the dartdoc area
• single clicking on symbols would update the contents of the panel
• clicking again on the 'toggle docs' button would hide the panel

[johnpryan]

Giving more control to the user is probably a good guiding principle here, so I'm leaning toward option 3 We could also try to see if option 1 is an improvement, but that could be done separately. Option 2 seems a little hard for the average person to intuit, but maybe we could make alt-clicking on a symbol enable the panel.