Synchronize selection between text editor and webview

[SReichelt]

Editing .slate files in the text editor would be easier if some info about highlighted text and items was exchanged between text editor and webview.

Idea:

• The current editor selection should always be highlighted in the webview as well as possible.
• When opening a menu or editing a variable name in the webview, the corresponding text should be selected in the editor.

This requires that the Slate instance running in the webview saves file ranges corresponding to all objects read from the file. These ranges must be updated when the file is changed by the webview itself.