Make a backup (filename.old) in editor when saving XML format scenes, layouts or materials, delete after successful save. This prevents losing the file if XML save crashes. Closes #530.
Revert "Allow UIElements to keep their current size if greater than the calculated size during layout updates."

This reverts commit b97c8d4712957a3953fa06e15e89b00d5b15df6d.

FileSelector or the editor's hierarchy window would not resize to smaller properly if the amount of items in the list was being reduced. The MinSize attribute should be used to absolutely prevent layouting making an element smaller.
For Travis CI - switch CI build to use OS X build environment.