TextEditor: add text cursor to assist interface

This will allow us to request assistance for a specific selection
instead of just the position.

Change-Id: Ib8e5b32d4a8f2936e5a6f1b7ac968d7f1d8d9de6
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/glsleditor/glsleditor.cpp

@@ -369,8 +369,7 @@ AssistInterface *GlslEditorWidget::createAssistInterface(
     AssistKind kind, AssistReason reason) const
 {
     if (kind == Completion)
-        return new GlslCompletionAssistInterface(document(),
-                                                 position(),
+        return new GlslCompletionAssistInterface(textCursor(),
                                                  textDocument()->filePath(),
                                                  reason,
                                                  textDocument()->mimeType(),