Improve UI for the child popups

[mdouchin]

At present, the children popups are displayed under the parent popup (which is ok), but are listed one after the other. Drawbacks:

• When there is more than 2 children, when need to scroll to get the content, and lose the visibility of the parent
• When there are more than one parent popup visible, it creates a long vertical list of items and it is not very easy to know which child belongs to which parent
• When there are more than one child layer for the parent, it is worse

Proposal: like QField does, replace the inline display

by a "card" type display,

• show only the first card (child popup)
• above, add a combo box allowing to select one of the children. The label of each combobox item could be the "Display field" computed by QGIS or at least a field chosen by the Lizmap editor
• two button Previous / Next
• a counter showing "3/25"

cc @nboisteault

[aeduard]

We are using child popup for displaying lithology for boreholes. All related lines from the child table should be visible for selected borehole (parent point layer). We are using the Compact tables view. That representation should be kept, it is really useful. Also as an enhancement a possibility for child display style to be set from the lizmap qgis plugin for each layer could be useful. In our case Compact tables view would be the preferred type.