textbox focused border can now be disabled

[ashbob999]

Added a new function to textbox and text_editor so that the border that is shown when the widget is clicked on can be disabled.
The need for this was discussed in #656.

Example use to disable it:

form fm;
textbox tb{ fm };

tb.enable_border_focused(false);

The name of the function may need to be changed though, to maybe something like enable_focused_border_when_clicked, so it's more clear.