Update JSON UI schemas (#192)

* Implement some missing `ui/` schemas

- Add `allow_debug_missing_texture`
- Add `propagate_alpha`
- Add `use_child_anchors`

* Tweak descriptions of `item_ref`

It should be noted that UI are referred to as "elements."

Moreover, the examples used colon (`:`) instead of dots (`.`) which contradicts the given description.

* Add more enum values to the `easing` property

These are all known values, aside from the five that was given.

source/resource/ui/ui.json

@@ -39,7 +39,7 @@
         "initial_uv": { "$ref": "./elements/properties/initial_uv.json" },
         "next": { "$ref": "./elements/properties/next.json" },
         "play_event": { "$ref": "./elements/properties/play_event.json" },
-        "propagate_alpha": { "$ref": "./elements/properties/alpha.json" },
+        "propagate_alpha": { "$ref": "./elements/properties/propagate_alpha.json" },
         "reversible": { "$ref": "./elements/properties/reversible.json" },
         "scale_from_starting_alpha": { "$ref": "./elements/properties/scale_from_starting_alpha.json" },
         "to": { "$ref": "./elements/properties/to.json" },
@@ -62,6 +62,7 @@
       },
       "properties": {
         "allow_clipping": { "$ref": "./elements/properties/allow_clipping.json" },
+        "allow_debug_missing_texture": { "$ref": "./elements/properties/allow_debug_missing_texture.json" },
         "allow_scroll_even_when_content_fits": { "$ref": "./elements/properties/allow_scroll_even_when_content_fits.json" },
         "alpha": { "$ref": "./elements/properties/alpha.json" },
         "always_handle_pointer": { "$ref": "./elements/properties/always_handle_pointer.json" },
@@ -183,7 +184,7 @@
         "primary_color": { "$ref": "./elements/properties/color.json" },
         "progress_control": { "$ref": "./elements/properties/progress_control.json" },
         "progress_hover_control": { "$ref": "./elements/properties/progress_hover_control.json" },
-        "propagate_alpha": { "$ref": "./elements/properties/alpha.json" },
+        "propagate_alpha": { "$ref": "./elements/properties/propagate_alpha.json" },
         "property_bag": { "$ref": "./elements/properties/property_bag.json" },
         "radio_toggle_group": { "$ref": "./elements/properties/radio_toggle_group.json" },
         "render_game_behind": { "$ref": "./elements/properties/render_game_behind.json" },
@@ -258,6 +259,7 @@
         "unchecked_locked_control": { "$ref": "./elements/properties/unchecked_locked_control.json" },
         "unchecked_locked_hover_control": { "$ref": "./elements/properties/unchecked_locked_hover_control.json" },
         "use_anchored_offset": { "$ref": "./elements/properties/use_anchored_offset.json" },
+        "use_child_anchors": { "$ref": "./elements/properties/use_child_anchors.json" },
         "use_last_focus": { "$ref": "./elements/properties/use_last_focus.json" },
         "uv_size": { "$ref": "./elements/properties/uv_size.json" },
         "uv": { "$ref": "./elements/properties/uv.json" },