remove "Joints" from sequencer in SK projects
fixed actual (and possible) crashes accessing index 0 of potentially empty arrays
fix relative position type, and changes in position type for SK
fix contentSize editable
updated SK default project with label position centered on % of parent container
added zPosition property to nodes
update SK default project with zPosition usage
fix % relative position not calculated for some node types
fixed saving changes position slightly (of children with % positioned parent)
updated SK default project
update SB+SK submodule ref
tryfix undo changing position (display only)
test proj
SK default project: bg color scales with content size
fix undo changing display position of child nodes
sk default project updated
update SB+SK submodule
fix possible crash when updating SK node positions
Merge branch 'develop' of github.com:apportable/SpriteBuilder into develop-steffen

Conflicts:
	SpriteBuilder/SpriteBuilder.xcodeproj/project.pbxproj
removed "experimental" from "New Sprite Kit Project" menu item