Add new symbol list category "Extern variables"

This is a requirement for an upcoming Python parser fix. This new category
will be currently only used by Python, C and D parsers. Before this change,
in C & D extern variables were sorted into the category "Other", now they
have their own category.
Set tag kind for Python imports to externvar and map them in the symbol list.

This fixes 'Go to Tag definition' for parsed Python imports as before the import
statement was chosen as the definition while we prefer the class definition as
target.