GridWrap: Add StretchItems option to fill available width

[alexballas]

Problem: Applications using GridWrap cannot stretch items to fill available width without causing scrollbar jitter during resize. The issue is that GridWrap uses CreateItem().MinSize() for both column count calculation and item sizing. When MinSize depends on viewport width (for stretching), a feedback loop occurs:

1. Viewport width changes
2. MinSize returns different stretched width
3. Column count recalculates
4. Content height changes
5. Scrollbar toggles on/off
6. Viewport width changes → goto 1

Proposed Solution: Add a StretchItems bool field that, when enabled, calculates stretched item widths in updateGrid() at layout time:

This keeps column count calculation stable (uses base itemMin) while still filling available width visually.

The StretchItems feature solves a fundamental limitation in GridWrap where stretching items to fill width and having stable scrollbars were mutually exclusive. By moving the stretch calculation to layout time, applications get both benefits without any trade-offs.