Add option to configure file line ending for an open document

Add into the text editor toolbar a combo box which allows to change
the line endings for an open document.
The supported line endings are LF and CRLF.

Change-Id: I029ba7867e0087b162edba7aba1bd1d1e966fd69
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/texteditor/texteditor.h

@@ -381,6 +381,8 @@ public:
     void unfold();
     void selectEncoding();
     void updateTextCodecLabel();
+    void selectLineEnding(int index);
+    void updateTextLineEndingLabel();
 
     void gotoBlockStart();
     void gotoBlockEnd();