Clipper: bunch of rework. (ocornut#3841, ocornut#1725)

- Focused/NavId now always included in display range.
- Any number of steps (while preserving zero-alloc policy).
- Non contiguous ranges for nav processing
- Moved new fields internally (+ moved StepNo away from sight so it doesn't get missused).
- Generally tweaks/refactors.

docs/CHANGELOG.txt

@@ -41,7 +41,12 @@ Other Changes:
 - Added IsMouseTripleClicked() function. Tracking multi-click count in IO structure. (#3229) [@kudaba]
 - Modals: fixed issue hovering popups inside a child inside a modal. (#4676, #4527)
 - Fixed IsWindowFocused()/IsWindowHovered() issues with childs inside popups. (#4676)
-- Clipper: Fixed content height declaration slightly mismatching the one of when not using a clipper.
+- Clipper: currently focused item is automatically included in clipper range.
+  Fixes issue where e.g. drag and dropping an item and scrolling ensure the item source location is
+  still submitted. (#3841, #1725) [@GamingMinds-DanielC, @ocornut]
+- Clipper: rework so gamepad/keyboard navigation doesn't create spikes in number of items requested
+  by the clipper to display. (#3841)
+- Clipper: Fixed content height declaration slightly mismatching the value of when not using a clipper.
   (an additional ItemSpacing.y was declared, affecting scrollbar range).
 - Nav: pressing PageUp/PageDown/Home/End when in Menu layer automatically moves back to Main layer.
 - Nav: fixed resizing window from borders setting navigation to Menu layer.