Bugfixes and enhancements for store_editor, json & commands

This are changes I did for Roger, he is contributing the changes to you, thanks to him.

* Reverted json library EasyJson back to smjansson (EasyJson has too many bugs)
* Removed command "store_hideownitems" and added ConVar "store_showownitems" (positive namings are more clear)
* store_editor: Changed position menu to negate item positioning when holding the shift key
* store_editor: Cleaned up the menu
* store_editor: Added custom bind +store_editor_bigchange for changing item positions by 5.0 units instead of the default 0.5
** You have to hold the custom button
* store_editor: Removed round restart and automatic item reloading (last one only when other players are still in edit mode)
* store_editor: Thirdperson mode is automatically activated when editing an item (be sure to set mp_forcecamera to 1 to be able to move the camera)
* store_editor: own items can be seen automatically when editing
* store_editor: Command is now executable without arguments (default: self), cleaned up code
* Added <smlib> as dependency (for thirdperson)