Added basic object selection and texture.
Moved object click detection from process_inputs() to handle_mousedown_object().
Made the clickable area a bit larger in the world.
Added ObjectBase.position_on_layer[] and the update of value in process_objects() to avoid multiple calculation of this value.
Added _2d_add() and _2d_subtract() helpers to ease the operations with coordinates.
Added mouse selection handling.
Disabled scrolling while selecting (TODO: fix this).