Various enhancements for the store_editor

* Created cvar "store_showownitems" and removed command "store_hideownitems"
* When editing a playermodel, you now get switched to thirdperson automatically and you can see your own items while editing
* Prevent reloading on items on save when another player is still editing (you are told who it is)
* Added new dependency "smlib"
store_editor: Made command executable without any arguments

No argument means self-targeting