Scrollable Headers and Footers

[davideas]

New big functionality to be added.
Scrollable Headers and Footers are special items that scroll along with all others, but they don't belongs to main items (business items) and they are always handled by the adapter beside the main items. Those items should be persistently located at the first and last positions:

• after refreshing the list.
• after filtering (not when searchText is filled, they should not be filterable).
• after a screen rotation with selected items.
• during the endless scroll / loading more: progressItem becomes a footer item but it will be always displayed between the main items and the others footers.

Headers and Footers should also have the following characteristics:

• cannot be selectable nor draggable.
• cannot be inserted twice, but many can be inserted.
• can be added and removed with certain delay without impacts for the current selection.
• can be of any type (also expandable) so they can be bound at runtime with any data inside. When expanded, subItems will become headers or footers too.
• won't be filtered because they won't be part of the main list, but added separately at the initialization phase (basically, implementing IFilterable will be useless).
• adding, updating, removing, moving and swapping main items with specific positions, must always be done via previous call getGlobalPositionOf() as usual.
• a new function to get the position of a main item without scrollable headers count.
• showing / hiding headers should continue to work.
• (postponed) if possible the scrollable headers can switch to sticky headers as well, if the option is enabled.
• (to evaluate!) the size of onUpdateEmptyView() parameter should represents the main item count.
• dragging main items should not impact the headers and footers.