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.

[TaiseiHamaya]

I believe that all issues I was able to identify on our side have been fixed in the previous commit. If there is anything else that needs to be addressed, please let me know.

[ocornut]

Apologies I didn’t have time to look at this yet but I will eventually. Thank you!

[ocornut]

The 4 calls to RenderWindowOuterSingleBorder are equivalent to simply calling AddRect() once.

[TaiseiHamaya]

Thank you for a feedback!
I have fixed to use AddRect() and the condition for border rendering.

Due to a breaking change in AddRect in v1.92.8, merging as-is will result in an error.
Since updating the version on the current branch would require a large number of commits, I am pushing it as v1.92.6 WIP.