DSL: support #INCLUDE directive

[soshial]

This directive in the beginning of DSL file joins the main DSL file with auxiliary DSL files.

Absolute path: #INCLUDE "c:\\Lingvo\\Dictionaries\\rus-rus_synonyms-2.dsl"
Relative path, same folder: #INCLUDE "rus-rus_synonyms-2.dsl"
Relative path, subfolder: #INCLUDE "Extra\\rus-rus_synonyms-2.dsl"
Relative path, parent folder: #INCLUDE "..\\Thesaurus\\rus-rus_synonyms-2.dsl"

[ilius]

Is the included dsl injected before or after the current dsl entries?

[soshial]

Does really order matter for DSL dictionaries? I think that we can just prepend glosses/entries in the beginning. Whatever is more convenient in the code.