Guard against crashing plugins

If a plugin crashes, ask the user if that plugin should be disabled.
For this track which plugin currently is changing its state. Remove that
information if the state change was successful.
At startup check if there is a plugin for which we wrote, but not
removed that information.

This is especially interesting if the user installed 3rdparty plugins.

Change-Id: I5729aa5c786b653d5bd53304f4fbeaca35ec9e71
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/app/main.cpp

@@ -680,6 +680,7 @@ int main(int argc, char **argv)
         }
     }
 
+    PluginManager::checkForProblematicPlugins();
     PluginManager::loadPlugins();
     if (coreplugin->hasError()) {
         displayError(msgCoreLoadFailure(coreplugin->errorString()));