Replace SplitPane with BSplitView

[diversys]

/boot/system/develop/headers/os/interface/SplitView.h

[augiedoggie]

I did start working on converting the main window to layout kit at one point, which includes changing to BSplitView. The other views(toolbar/infobar/listview) didn't behave properly. They'll need to be fixed or replaced with equivalents from Haiku.