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/pythoneditor.cpp

@@ -25,6 +25,7 @@
 
 #include "pythoneditor.h"
 
+#include "pyside.h"
 #include "pythonconstants.h"
 #include "pythonhighlighter.h"
 #include "pythonindenter.h"
@@ -118,6 +119,7 @@ public:
             return;
 
         PyLSConfigureAssistant::instance()->openDocumentWithPython(python, this);
+        PySideInstaller::instance()->checkPySideInstallation(python, this);
     }
 };