Fixes #9137
Move scroll bar drawing into HaikuControlLook
Added B_SCROLLABLE flag to BControlLook
Update FakeScrollBar in Appearance to also draw using HaikuControlLook.
Focus works on scroll bars again, used by FakeScrollBar... and probably
nowhere else.
Added private _ScrollingEnabled() convenience method to BScrollBar and
use it in a few places making.
Create ScrollBarPrivate.h header to share a couple of scroll bar related
enums with HaikuControlLook that come from BeOS Scroll Bar prefs.
Stuff arrow_direction enum into BScrollBar::Private as it has been
succeeded by a similar enum already present in BControlLook and is only
around now for BScrollBar::Private::DrawScrollBarButton.
Change-Id: Idc31ee41de091ba45ded2f0315a004af00143803
ec1b18c58a