Update README.markdown
Improved the description of the sample in the README
[WPF] Fix bitmap conversion issue
[Mac] Fix widget size calculation when margins are set
Add support for getting screen geometry information
Add field binding check

When binding a field to a list or tree column the field must already
belong to a store or data source. This is required in the gtk backend,
since the fields must have an index assigned when creating the column
mappings.
Merge branch 'master' into xwt-next