Feature: More flexible export to SVG / Code

[Undistraction]

Feature

Currently the export functionality is pretty limited. A user has to create a grid at exactly the right dimensions which can be fiddly. It should be easy to scale up or down an export to the designed dimensions in the export dialogue.

Implementation

Assuming no change to aspect ratio, this should be easy to achieve by allowing a user to set the desired width or height and scaling the other value up using the aspect ratio of the grid.