[Mac] Fix the background/foreground colors when selecting cells
[Mac] Fix horizontal scrolling in ListBoxBackend

By default, the NSTableColumn is sized to the width of the
NSTableView, and horizontal scrolling is based on the column width,
not the content. This patch calculates the width of all the rows
and manually sets the column width so horizontal scrolling works
as expected.
[Mac] Implement RowActivated event on ListViewBackend
[Mac] The docs say to use ClickedRow instead of SelectedRow

https://developer.apple.com/library/mac/documentation/Cocoa/Reference/ApplicationKit/Classes/NSTableView_Class/index.html#//apple_ref/occ/instp/NSTableView/doubleAction
Merge branch 'master' into xammac-unified