File/Folder delete improvements

When deleting a file with unsaved contents, the user is now prompted to keep the editor open or to close it and loose changes. This dialog appears if you delete a dirty file directly _or_ delete a folder containing a dirty file.

Adding this notification actually _simplified_ the code because we now rely on `FileSyncManager.syncOpenDocuments()` to take care of files that were deleted from disk. There is one small side affect: this call will also sync any other changes from disk into any open editors. This is an extreme edge case, and even if it _does_ occur, the behavior is technically correct. This is what the user would see anyway if Brackets was deactivated and reactivated.

This pull request also adds unit tests for the `fs.moveToTrash()` function.