Python: Check for Python language server after document was opened

Show an info bar entry with a one click solution to setup a language
server if the python which is most likely to be used for this file
has an installed language server.

Change-Id: Ia52bb043b543699527740951f68cc6be546833df
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/python/pythonrunconfiguration.h

@@ -42,6 +42,7 @@ class PythonRunConfiguration : public ProjectExplorer::RunConfiguration
 
 public:
     PythonRunConfiguration(ProjectExplorer::Target *target, Core::Id id);
+    QString interpreter() const;
 
 private:
     void doAdditionalSetup(const ProjectExplorer::RunConfigurationCreationInfo &) final;
@@ -49,7 +50,6 @@ private:
     bool supportsDebugger() const;
     QString mainScript() const;
     QString arguments() const;
-    QString interpreter() const;
 
     void updateTargetInformation();
 };