Support large scroll offsets

[dhardy]

Currently, scroll offsets are limited by the i32 type to 2^32, and since #221 further limited to approx 2^24 to allow precise representation via f32. For most things this is fine but for ListView (and MatrixView etc.) this may be limiting.

We should be able to fix this by placing widgets nearer the origin in these view widgets, thus ScrollComponent's offset would be replaced with scroll_offset (position used by scroll bar) and offset (used to adjust draw position and events and much more limited in range, possibly calculated as scroll_offset % (1<<20) or similar).

Additionally we could then use i64 or f64 for scroll_offset and in Scrollable and ScrollBar, allowing scrolling over much larger ranges.