Added CurrentEventRow to TreeView.
This allows e.g. Toggled event handlers to know which row was changed.
It is also implemented for the Text cell renderer text changed events.
The Image and custom cell renderers were also changed to receive the TreeViewBackend, but do nothing with it at this point.
Other tree events (such as selected row changed) do not set the CurrentEventRow property.
This property is only implemented for the Gtk backend.
Merge remote-tracking branch 'origin/master' into luiscubal-tri-state-cell-view
Xwt.sln: MacGtk test no longer included in Linux-Debug configuration.
Updated Checkbox State API.
Samples: Added event handlers to Checkboxes.
Added tri-state checkbox cell view API and implemented GTK backend.
Xwt: Made CheckBoxStateUtils internal
Merge pull request #233 from mono-soc-2013/luiscubal-tri-state-cell-view

Tristate checkboxes
[Tests] Don't add the test assembly if it has already been provided
[Mac] Track api changes in checkbox
[WPF] Track api changes
[Mac] Allow setting the mixed state even when AllowMixed=false
[Mac] Add support for State field to checkbox cell view
[GTK] Fix icon rendering issue

Use the standard RenderIcon method when hi-res icon support is
not available
Added new overload for Image.FromResource
[GTK] Fix ComboBoxEntry issue

Always set the text column. Don't clear the ComboBoxEntry
on creation, since that destroys the default cell renderer.
[WPF] Fix AppendPath

The current path was appended, but not the already closed figures.