Blender Git Loki

UI: Refactor how dragging onto text buttons works, fixing issues

There was a bunch of special handling to support dropping data-blocks onto
string or search-menu buttons, to change the value of these. This refactor
makes that case use the normal drop-box design, where an operator is executed
on drop that gets input properties set by the drop-box. This should also make
it easier to add support for dragging assets into these buttons.

In addition this fixes an issue: Two tooltips were shown when dragging assets
over text buttons. None should be shown, because this isn't supported.