UI: Allow Changing Editor Outline Width

The gaps between editors need to be selected and manipulated in order
to resize areas. How wide these need to be depends on the accuracy and
resolution of your pointing device. And also on the fine motor control
and visual acuity of the user. If you are using a tablet pen, touch
device, or are visually or physically challenged, then you need these
gaps to be wider. This PR allows a wide latitude in this.

Pull Request: https://projects.blender.org/blender/blender/pulls/126334

release/datafiles/userdef/userdef_default.c

@@ -66,6 +66,7 @@ const UserDef U_default = {
 
     .ui_scale = 1.0,
     .ui_line_width = 0,
+    .border_width = 2,
 
     /** Default so DPI is detected automatically. */
     .dpi = 0,