Terminal: Implement selection via double click

Change-Id: I7665df5f37836331f202f168828bb1b1636bf5de
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/terminal/terminalwidget.h

@@ -162,6 +162,10 @@ private:
     Utils::Terminal::OpenTerminalParameters m_openParameters;
 
     QDateTime m_lastFlush;
+    QDateTime m_lastDoubleClick;
+    bool m_selectLineMode{false};
+
+    std::u32string m_currentLiveText;
 };
 
 } // namespace Terminal