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/python.qbs

@@ -9,6 +9,7 @@ QtcPlugin {
     Depends { name: "Core" }
     Depends { name: "TextEditor" }
     Depends { name: "ProjectExplorer" }
+    Depends { name: "LanguageClient" }
 
     Group {
         name: "General"