Terminal: use shell name as terminal tab name

This way cmd, bash, powershell are more descriptive than "Terminal"

Change-Id: I19310f423cd4188ecc48580a30ed414833a15aee
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/plugins/terminal/terminalwidget.h

@@ -52,6 +52,8 @@ public:
         QPoint end;
     };
 
+    QString shellName() const;
+
 signals:
     void started(qint64 pid);
 
@@ -119,6 +121,8 @@ protected:
 private:
     std::unique_ptr<Utils::QtcProcess> m_process;
 
+    QString m_shellName;
+
     std::unique_ptr<VTerm, void (*)(VTerm *)> m_vterm;
     VTermScreen *m_vtermScreen;
     QSize m_vtermSize;