Move sub context properties into own sub menu in PathPropertyDrawer

[Christian Oeing]

This makes nested contexts easier to navigate through.

Use "." or "this" or "#" or similar description to use context itself. Could also be used for root path so no ContextGetter is required there.

[Christian Oeing]

Sub menus already implemented. For self reference see issue https://bitbucket.org/coeing/data-bind/issue/9/allow-self-reference-of-context-in-path