Extend class diagram with new elements

[haydar-metin]

works on #70

Extends model with

• [x] EnumLiteral
• [x] Operation
• [x] DataType
• [x] PrimitiveType
• [x] Package

Improves architecture:

• [x] by using Context classes (removes passing the same parameters everywhere)
• [x] by using Base*SemanticCommand and Base*NotationCommand for better structure and code reusability
• [x] by using codecs for encoding and decoding requests between glsp server and model server instead of doing it manually
• [x] by using matchers for detecting usages of different elements for delete operations
• [x] by using a general approach for working with child relations and the necessary calculations (e.g., relative position)