Split mouseTripleClickEvent method into selectLine()

Use a helper method selectLine() which is most of the
mouseTripleClickEvent().  Also, in preparation for upcoming patch to
allow selectLine per shortcut.

CCBUG: 327395

src/TerminalDisplay.cpp

@@ -2615,7 +2615,12 @@ void TerminalDisplay::mouseTripleClickEvent(QMouseEvent* ev)
     int charLine;
     int charColumn;
     getCharacterPosition(ev->pos(), charLine, charColumn);
-    _iPntSel = QPoint(charColumn, charLine);
+    selectLine(QPoint(charColumn, charLine));
+}
+
+void TerminalDisplay::selectLine(QPoint pos)
+{
+    _iPntSel = pos;
 
     _screenWindow->clearSelection();
 

src/TerminalDisplay.h

@@ -658,6 +658,7 @@ protected:
     void clearImage();
 
     void mouseTripleClickEvent(QMouseEvent* event);
+    void selectLine(QPoint pos);
 
     // reimplemented
     virtual void inputMethodEvent(QInputMethodEvent* event);