Python: add PySide installation check on document open

Checks if the document imports PySide and whether the selected python
interpreter can find a PySide installation. If not show a global info
bar that can install PySide via pip like the python lsp server.

Task-number: PYSIDE-1742
Change-Id: I02c0d5f6eb268f3d8826d4fb9d9ec3c7c48b8638
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/python/pythonrunconfiguration.h

@@ -45,7 +45,7 @@ public:
     QString interpreter() const;
 
 private:
-    void updateLanguageServer();
+    void interpreterChanged();
 
     bool supportsDebugger() const;
     QString mainScript() const;