Allow titlebar_separator to use window border width, even when titlebar border is 0

[Nairou]

Currently, titlebar_separator determines whether the border separating the titlebar from the window content is visible. If it is visible, it uses the border width defined in titlebar_border_thickness. If titlebar_border_thickness is 0, then titlebar_separator has no effect. Depending on theme colors, this can lead to weird effects where the window has a border, but the titlebar does not, so the borders on the side just end at the top.

I would like the option to have the titlebar separator use the window border setting, so that it could optionally remain visible even when titlebar_border_thickness is 0.

Allowing the titlebar separator to use the window border would allow a continuous border around the window content, regardless of whether a border is specified for the titlebar. This could allow for some interesting effects, like when you need to see the window's title, but still want to get as close as possible to the non-titlebar look by keeping the title outside the window border.