Add configuration file that allows specifying a default model

[hannobraun]

When Fornjot is started without a command-line parameter to select a model, it loads a default model (currently models/cuboid). This is useful for development (cargo run without parameters results in something useful), but is just weird behavior if a Fornjot binary is run outside this repository:

• A user just starting Fornjot will get a confusing error message about some model they never heard of not being found.
• We might want to have different behavior for the "no parameters" case, like showing an "open file" dialog.

To address these problems, while at the same time preserving the ease of development, I propose adding a configuration file that allows for specifying a default model. I prefer TOML for these kinds of files. Here's what that could look like:

# The file could be called `Fornjot.toml`
default_model = "path/to/my-model"

Issues #38 and #56 are related.

[hannobraun]

In addition, it should also be possible to specify a default model path. For example, if the following were specified in Fornjot.toml:

default_path = "model"

Then fj-host -m my-model should load the model from models/my-model. This would further enable keeping the current behavior when running from this repository, without making behavior outside this repository weird and confusing.

[hannobraun]

I'm working on this.