Resolve global variables

[kornilova203]

Example:
Usage of a on second line should be resolved to the definition global a

global a;
a = 1;

To support actions for global variables such as 'go to declaration' or 'find usages' we need to modify resolve.
There is big article about resolve in IntelliJ IDEs: References and Resolve.

Usually we only need to add mixin for a declaration rule in bnf grammar e.g. mixin for class_declaration

After adding mixin the MatlabResolvingScopeProcessor will check these declarations during resolve process.

But there is a problem with global variables: one statement may declare multiple variables.
So we need to:

1. wrap each IDENTIFIER from global_variable_declarations rule in new global_variable_declaration rule
2. add MatlabDeclarationBase mixin to global_variable_declaration (similar to class_declaration)
3. add MatlabBlockMixin mixin to global_variable_declarations (it will pass resolve processor to each variable declaration)
4. write resolve tests (see ResolveTest)

Note: you don't need to resolve references from different files. It's a separate issue #24

[kornilova203]

Fixed by #65