FEATURE: Save/Open options from GUI

[ajjackson]

There are two save/load options that would be useful from the GUI:

• Loading a user-specific file with element settings (i.e. custom colours, radii...)
• Saving the settings for the current render

The GUI is already a little cluttered in terms of buttons, so it might be beneficial to put these in an application File menu.

The file format would naturally be the same .conf files as found in addons/vsim2blender. It isn't entirely clear how the two settings files should be related:

• Does the "current render" conf remember the preferred element settings file?
• Are the element settings dumped into the "current render" conf?

The difference between these in practice is what happens when the user saves a configuration and then modifies their personal elements conf.

[ajjackson]

Implemented, released in 1.0.0