Add examples illustrating Issue #169.
Toward resolving issue #163: `content` property naming conflict

Add `content` property where `children` were wrapping JFX `content`.
Mark that `content` usage ad deprecated. They will be eventully removed in ScalaFX 8 branch.
Merge remote-tracking branch 'origin/SFX-2'
Use `children` instead of deprecated `content` (Issue #163)