Add the variable module

[JoelNiemela]

The variable module should contain a way to inspect variables' values and modify them. Buttons related to variables, such as :, currently in the function module, should be moved to the variable module.

Things to look out for:

• Make sure that the variable inspector has enough screen space to display longer values, such as big numbers and complicated functions.
• There should be a way to browse existing variables without taking up too much screen space. Maybe this could be a stacked layout, togglable with a button?
• Constans could also be moved to the variable module. These might also need to be in a stacked layout to save on space, or they could use the bottom row of the variable-browser layout.