Support for managing imports

[SimonCockx]

Right now, managing imports is a manual process. E.g., if a modeller needs a symbol from another namespace, they will manually add an import to.my.namespace.* at the top of the file. This results in a series of issues:

• A lot of wildcard imports, even though a single identifier is needed.
• Duplicate imports to the same namespace which aren't detected. (there should be a warning)
• The imports are ordered arbitrarily, making it difficult to see whether or not a particular namespace is imported or not.

I would propose to add the following features:

• Import symbol action to automatically add an import to a symbol at the top of the file.
• Add warning on duplicate imports.
• Optimize imports action.

The Optimize imports action would do the following:

• If the number of symbols from an imported namespace is greater than a threshold (let's say 3), then replace those imports with a wildcard import. Otherwise, replace a wildcard import with the actual individual symbol imports.
• Sort the imports.
• Group the imports per top-level namespace. (e.g., drr.foo is a different group from cdm.foo, but the same as drr.bar)

Related: (resolving these issues will also help with managing imports)

• Content assist and validation for imports #121
• Importing the same identifier from different namespaces results in a Couldn't resolve reference to ... #486