Improve styling of user interface

[jakobaxelsson]

Use a CSS framework. Pico CSS appears to be a simple alternative mainly based on semantic tags. It can be customized e.g. to have a sparser layout by setting CSS variables (--spacing in the case of layout).

Main UI elements:

• Nav for a menu bar with dropdown menus. Menu items include File (loading and saving to file), View (customizing the view to show different things, and selecting what to show in the right portion of the field.
• To the left, a map area with simulation controls. Probably about 2/3 width on computer screen.
• To the right, an area with various input forms, such as for generating a new map or inspecting various elements of the simulation. What is shown could be controlled from the menu.

Some restructuring of the UI code is needed for this.

Also, the map area should be configured based on the available space given to it, and the actual map should be scaled to fit within that area.

[jakobaxelsson]

Did some experiments with Pico CSS, but not as easy as expected. In particular, the nav bar did not work at al. Have restructured the layout of the UI so that configuration controls are now to the right of the map.

[jakobaxelsson]

The SVG drawing now scales reasonably well to available space. Also, the drawing has been simplified so that one grid unit corresponds to one unit in the SVG's internal coordinate system. Scaling takes place on the entire SVG through CSS styles.

[jakobaxelsson]

A CSS file is now provided, where UI styling should be put.

[jakobaxelsson]

The menubar is in place. What remains is to save configurations from file and load them again.

[jakobaxelsson]

Fixed load and save of configuration files.