Toggleable left panel

[ghost]

Closed issue #138 reminded of my desire to be able to minimize the editor panel.
Being able to proofread my typeset document without the distraction of the editor
pane on the right makes my proofreading much easier. With a hidden editor pane
I can read through the document, press Ctrl/click in preview pane and then unhide
the editor pane to make a correction.

Using the code in commit bf88cbc for inspiration I was quite easily able
to make the desired change, and it works well for my purpose. I would be happy to share
my code, but I'm not sure how, and anyway it is very simple: just adding adding a field to
GummiGui, a method to git-menu.c, and a few lines to gummi.glade.

Thanks in advance for your consideration.

[ghost]

Attached is a diff file that (almost) implements requested feature, and CSV file of possible translations for the tooltips I added.
diff.txt
lang.csv

Thanks in advance for your consideration.