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/elements/properties/allow_debug_missing_texture.json

@@ -0,0 +1,5 @@
+{
+  "title": "Allow Debug Missing Texture",
+  "description": "If true, the element will show the black and purple placeholder texture if the referenced image is not found.",
+  "$ref": "../../general/boolean.json"
+}