Show/hide the side panel

Hello Denis,

Thanks for tatlin, its small, clean and does its job really great. 

As my primary use for tatlin is to use it as a pre-viewer for my OpenSCAD files, I don't really need the side panel. Even more, I don't want to accidentally change the STL file, so I tried to find a way to hide and unhide the panel (by introducing a "View" menu) as well as a command line argument (--hide-panel) to disable it upon starting up.

You might want to have a look at it to see whether it fits with your goals of tatlin.

This is my first ever pull-request and unfortunately not how it should have been done: I should probably have made the fix in a special topic branch, so there's some clutter lying around (like a shell script for starting tatlin and a few changes to the README).

Code-wise I'm semi-happy with my solution. The menu-generation is a bit inter-winded with the file menu, but I also didn't want to change too much on your code. Also the command line parsing is quite hacky and one should probably think of using `argparse`. But once again, I tried to change as little as possible.

Best regards,

Dinkel