BScrollView: Automatically update the scrollbar proportions in layout mode.

Since we know what size the target view is / wants to be, we can automatically
set the range, steps, and proportion trivia

BScrollView: Automatically update the scrollbar proportions in layout mode.

Since we know what size the target view is / wants to be, we can automatically
set the range, steps, and proportion trivially. In non-layout mode, we retain
the old behavior. Applications or views that need custom scrolling behavior almost
certainly will be using BScrollBars directly and not this, so this should not be
"wasted computation" in pretty much any case.

Greatly improves the appearance and UX of the default case of a layouted
view inside a BScrollView.

Change-Id: Ia6ff6ee14df96799c579e15d274fd4c849675577
Reviewed-on: https://review.haiku-os.org/c/892
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

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Fix typo: on => one

BScrollView::ScrollBar return val docs past tense

Clarify ScrollView::ScrollBar() direction parameter in docs

IK: Change orientation param name to direction

for ScrollView::ScrollBar in ScrollView.h and the documentation. This makes
the parameter name consisent and fixes a doxygen warning.

ScrollView docs: make parameter name match the header file.

IK documentation update

* Add \since directive to each method.
* Add documentation for BScrollBar and BScrollView classes.
* Title Case group titles.
* Some other minor documentation updates.