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/python.qbs

@@ -19,6 +19,8 @@ QtcPlugin {
         files: [
             "pipsupport.cpp",
             "pipsupport.h",
+            "pyside.cpp",
+            "pyside.h",
             "python.qrc",
             "pythonconstants.h",
             "pythoneditor.cpp",