demos: Fix `z-index` with fixed demo header
demos: Fix `z-index` with fixed demo header

Increasing dialog `z-index` for demo header use case,
stacking logic as follows:

1. Widgets values remain between `-1` & `3`,
2. menus and popups values remain at `4`,
3. assigning demo header and similar elements `5` and
4. increasing dialogs in demos only to `6`.

Bug: T166523
Change-Id: I24393ea2ba212baf2af142aadf0487ca1ed465bc

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui