Crossing project boundaries

[maraf]

Current implementation is limited to a project where the dialog was opened. We can't properly support #76 without crossing it.
To make it work correctly, we should

• Process relative paths based on tree in solution explorer.
• Replace checkbox "Is file" with radio list ("File", "Folder", "Solution folder").
• Based on current path (evaluated as user types), we should enable/disable individual item types.