Add a property/client-attribute to specify a minimum z-index for popups and modal windows/spanels to allow layout integration with existing z-index styles.

If multiple popups stack up the next higher z-index should be chosen.

e.g. 3 stacked modals

1. modal with default
2. modal with min-z-Index 5000
3. modal with default

result

1. modal z-index: 1800
2. modal z-index: 5000
3. modal z-index: 5001