IDocument: Simplify permission changes

Take care of handling file permission changes centrally.

TextDocument had its own, caching implementation of tracking the backing
file's read-only state. Move that into IDocument directly.

IDocument::reload with a permission-only change is not a very
interesting case, but every subclass needed to add handling of it.
Instead, remove TypePermission from the file-change types, and handle it
separately via the now unified checkPermissions() implementation.
IDocument::reloadBehavior already was never called with TypePermission.

Change-Id: I321d47ba6193bc878efa9bb50ba7a739fa492745
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/designer/formwindowfile.cpp

@@ -225,9 +225,6 @@ bool FormWindowFile::reload(QString *errorString, ReloadFlag flag, ChangeType ty
         if (!wasModified)
             updateIsModified();
         return true;
-    }
-    if (type == TypePermissions) {
-        emit changed();
     } else {
         emit aboutToReload();
         const bool success
@@ -235,7 +232,6 @@ bool FormWindowFile::reload(QString *errorString, ReloadFlag flag, ChangeType ty
         emit reloadFinished(success);
         return success;
     }
-    return true;
 }
 
 void FormWindowFile::setFallbackSaveAsFileName(const QString &fn)