Add Help button style with Mac implementation
[Mac] Don't display text over disclosure buttons

Cocoa will display the text even when the bezel style makes it look weird.
Behave the same way as the Gtk backend and don't display the label when
the button type is disclosure.
[WPF] Fix drag&drop issue

Do an additional DragOver check before dropping, since the Drop event may
come without a DragOver. Also properly initialize the initial drag area.
[Xwt.Mac] Make links in labels have the correct style and be clickable.

Known issue: as described in, for instance, http://www.bdunagan.com/2010/07/10/cocoa-tip-urls-in-nstextfields/ link created in bare NSTextField have several cosmetic issues with font and hand cursor until you click them for the first time. This needs to be addressed separately.

Ref: https://developer.apple.com/library/mac/qa/qa1487/_index.html
Fixing Issue #235 TreeView problem with overwriting selected values at Loaded event with default Items[0] value and not selecting TreeItem because Item was selected before TreeItem was because WPF is LazyLoading TreeItems for Items after Loaded event...