Improved editor control

[dkastl]

Fixes #310 .

Changes proposed in this pull request:

• Adds a new control button Edit
• Enables vertex and line snapping
• Inform the user about ALT+<click> shortcut to delete a vertex.
• Provides a delete button through tooltip
• Draws existing features temporarily light gray when adding a new feature

[sanak]

@dkastl Thanks for improvement!

About tooltip message, I added comment on the line.

Also, if the followings are possible, it would be nice, I think.

• Disable Edit button when:
  • There is no geometry on the map
  • Geometry type is point (point seems to needs creating new one at this moment)