TreeView: CheckBox support

[leviwilson]

TreeView controls have the ability to add checkboxes to them. It would be nice to have the ability to check / uncheck items based on their index or value.

class SomeScreen
  include Mohawk
  window title: /Whatever/

  tree_view(:users, id: 'usersTreeView')
end

s = SomeScreen.new
s.check_users 'First User', 'Second User' # => selects "First User" and "Second User"
s.uncheck_users 'Second User' # => unchecks the "Second User" item

[leviwilson]

@jacob-ewald not sure when I'm going to get to this, but just wanted to track it.