CSSTUDIO-1976 Escape newlines in TextField "text" to prevent newlines from being removed in Label widgets.

[abrahamwolk]

This merge-request adds a mechanism that escapes newline characters from the TextField associated with the text of Label Widgets under widget Properties.

It fixes an issue that can be reproduced as follows:

1. Create a Label widget.
2. Enter a text that contains newlines, such as (by clicking on the "Open Editor" button with label "..."):

Line 1
Line 2
Line 3

3. Click anywhere else in the OPI, so that the Label widget is now de-selected.
4. The newline characters of the entered text have now been removed, and the text of the Label widget is now:
   Line 1Line 2Line 3

I have tested the fix under Linux and Windows.

[kasemir]

Good one!