Alpha 20: Checkboxes and Batch Editing

bug fixes, improved object selection

Engine Changes:
-changed how the cursor records highlighted objects
-cursor can tell if ui objects are clicked/not clicked
-added enable/disable functionality to clickable objects
-fixed dropdown menu buttons
-simplified "mouse_function" methods

Demo Changes:
-fixed menu button
-fixed selection box
-multiple objects can be deleted at once
-all interactive elements are disabled when game is paused
-added a "create object" button in order to expedite the process
-updated readme