[zenmap][GUI] added topology legend button.

[surajhande]

As suggested in the issue #258 by @dmiller-nmap I added a topology "legend" button to the Toolbar. Also added an image
(legend.png) to the pixmaps folder

[dmiller-nmap]

@surajhande555 Thanks for contributing! I guess I should have been clearer with the requirements for this option. I don't think a static image will be sufficient, since we need it to look good in varying screen resolutions/sizes and to be easy to update when/if we change the images. Check back on #258 soon and I will put some notes there.

[surajhande]

@dmiller-nmap I've written a LegendWindow class with all the shapes and labels drawn independently as you said.
Please review and let me know.

[dmiller-nmap]

@srjhnd Thanks so much for doing this! I made a few changes to bring it in line with our code conventions, but overall this is your code and you get all the credit. Changes:

• Fixed whitespace issues (trailing, tab indents instead of spaces)
• PEP8 warnings addressed
• Factored out some common drawing code into separate functions to clarify and avoid errors
• Relative layout by adjusting y-coordinate makes it easier to add new elements
• Support for translation by wrapping translatable strings in _()
• Minor layout adjustments