Provide conversions between softlines and buffer positions
Use softlines instead of hardlines for accessibility

Fixes #1149.

VoiceOver expects AXLineForIndex and AXRangeForLine to be consistent with
the movement by line using arrow up/down. That means it expects "line"
to mean softline.

VoiceOver also distinguishes commands to move by:
* paragraph (hardline): VO-Shift-PgUp/PgDown
* line (softline): VO-up/down

These are available when interacting with the text area (VO-Shift-down).

This commit therefore fixes the AXLineForIndex and AXRangeForLine
to make "line" mean "softline" instead of "hardline" as was the case
until now.
Fix accessibility visual bounds ending at line boundary

Accessibility visual bounds for a range of a single softline should always
be also visually restricted to the softline. This is in order to be consistent
with implementation in Apple's apps (e.g. TextEdit).