Fix for some edge cases

[alexk111]

The PR covers two edge cases for some dialogs triggering an unwanted minimize:

1. Overwrite dialogs in Firefox. Reproduced by Right-click on Image > Save image as > Pick an existing file. Fixed with the if (!windowB.closeable) check.
2. Overlay windows in Krita. Reproduced by clicking any of these on the Top bar: Fill Gradients, Fill Patterns, Choose Brush Preset. Fixed with the ancestor comparison for both windows: getAncestorForTransientWindow(windowA) === getAncestorForTransientWindow(windowB)