add socket activation for extensions (#118)

Option `X-AW-Socket` is used. Any message received by this socket will
trigger extension update. If the option is set, `X-AW-Interval` will be
ignored

sources/awesomewidgets/extscript.h

@@ -73,11 +73,13 @@ public slots:
     void writeConfiguration() const;
 
 private slots:
+    void startProcess();
     void updateValue();
 
 private:
     QProcess *m_process = nullptr;
     Ui::ExtScript *ui = nullptr;
+    bool canRun() const;
     void translate();
     // properties
     QString m_executable = QString("/usr/bin/true");