Add TreeSortable
Merge pull request #169 from idealeric/sortable

Add TreeSortable
Add Container's ResizeMode. Closes #171
Remove comment
Fix a Reference/Memory Bug in TreeSortable

This commit ensures that a reference to the sort function remains in go,
so that the garbage collector doesn't free the space.
Merge pull request #172 from idealeric/sortable

Fix a Reference/Memory Bug in TreeSortable
Fixes #174