Opening multiple YesNo Dialog boxes doesn't work properly

[ianling]

Describe the bug
HellSpawner uses the OpenDiablo2/dialog package for displaying some dialog boxes. The bug occurs when two things try to open a Yes/No dialog box at the same time.

This can occur in HellSpawner when you have two editors open that both contain unsaved changes, and then you attempt to close HellSpawner. Both editor windows will attempt to prompt you to save your unsaved changes, but only one dialog box appears. Selecting either Yes or No will cause your selection to affect both editors.

To Reproduce
Steps to reproduce the behavior:

1. Open up the Project Explorer.
2. Right click in the Project Explorer and create two new fonts.
3. Click on each font to open editors for them.
4. Set one of the fields in each font. It doesn't matter which field.
5. Without doing anything else, try to close HellSpawner.
6. A dialog box appears asking if you would like to save your unsaved changes.
7. Select Yes. No other dialog box appears.
8. HellSpawner should be closed at this point. Re-open HellSpawner.
9. Notice that both of the editors saved changes, despite you only being prompted to save changes in one.

Expected behavior
A separate dialog box should appear for each editor that contains unsaved changes. For example, you might have three editors open with unsaved changes in each of them, but you want to discard the changes in two of them and only save in one of them.

Other info
This bug is technically in the OpenDiablo2 repo, but it is causing an issue in HellSpawner, so I wrote this up here.

We should either fix the bug in the other repo, or do something in giu/imgui instead of using dialog boxes like this.