Add a feature to draw a border around the selected window

[TaiseiHamaya]

I have implemented, in the docking branch, the option proposed in issue #3537 to draw a border when a window is selected.

The intent of this change is to provide an option that allows the selected window to be emphasized not only via ImGuiCol_TitleBgActive, but also through a border, similar to the behavior in Visual Studio 2026.

Please refer to the attached GIF for the behavior after the change.

[TaiseiHamaya]

With the current implementation, I have noticed an issue where, when it overlaps with a normal border or a resize border, two lines end up being drawn.
If necessary, I am considering making improvements; however, resolving this would require fundamentally reworking the decoration handling of docked windows, which I expect would take a considerable amount of time. For now, I plan to try various approaches and explore a better implementation.