Add properties/items to project file when generating template

At the moment the template engine is not capable of modifying any existing files, including project files.
This is an issue for certain class of templates, requiring update of project files in addition to adding new files.

As concrete case, this is required for F# item templates. When adding item template to F# project, it has to be added to project file as well:
```



```

Therefore, to support F# item templates, the engine should be extended to allow project file modification.

Suggested basic F# item templates at the moment:
- F# file - generates empty *.fs file and adds it to project
- F# signature file - generates empty *.fsi file and adds it to project