QtFRED dialog delete-on-close

[Goober5000]

From PR #6322:

Set the delete-on-close attribute via diag->setAttribute(Qt::WA_DeleteOnClose);

Many dialogs in qtFRED are allocated with new but never freed with delete, causing memory leaks until the program is closed. Adding delete-on-close should fix the memory leaks, according to what I've found by searching online.

[Goober5000]

@TheForce172 It looks like I can't actually assign a ticket to you until you have some activity on it. Post a comment and that should be enough, as it was on #6456.

[TheForce172]

That seems a bit unintuitive design.

[Goober5000]

¯⁠\⁠_⁠(⁠ツ⁠)⁠_⁠/⁠¯