Integrate file and code browser for submissions

Resolves #124 (closed).

• Zip solutions can be loaded
• Folders with children can be opened
• Folders without children "act" as disabled elements, but shown as locked folder
• Some languages have icon and syntax support
• The treeview and codeview have fix height and can be scrolled seperately

Possible future issue/improvement:

• if the application will handle single file uploads, then it should be refactored to accept these kind of files
• add theme selection for code viewer (themes)
• if a file is too big then it can take a lot of time to load it, so probably some kind of restrictions to not allow these files (for example .jar)