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"
+}

source/resource/ui/elements/properties/easing.json

@@ -3,7 +3,40 @@
   "description": "The easing of the animation.",
   "anyOf": [
     {
-      "enum": ["spring", "in_sine", "linear", "in_quint", "out_cubic"]
+      "enum": [
+        "linear",
+        "spring",
+        "in_quad",
+        "out_quad",
+        "in_out_quad",
+        "in_cubic",
+        "out_cubic",
+        "in_out_cubic",
+        "in_quart",
+        "out_quart",
+        "in_out_quart",
+        "in_quint",
+        "out_quint",
+        "in_out_quint",
+        "in_sine",
+        "out_sine",
+        "in_out_sine",
+        "in_expo",
+        "out_expo",
+        "in_out_expo",
+        "in_circ",
+        "out_circ",
+        "in_out_circ",
+        "in_bounce",
+        "out_bounce",
+        "in_out_bounce",
+        "in_back",
+        "out_back",
+        "in_out_back",
+        "in_elastic",
+        "out_elastic",
+        "in_out_elastic"
+      ]
     },
     {
       "$ref": "../../general/item_ref.json"

source/resource/ui/elements/properties/propagate_alpha.json

@@ -0,0 +1,5 @@
+{
+  "title": "Propagate Alpha",
+  "description": "If true, this element will apply the same alpha value to its controlled element(s).",
+  "$ref": "../../general/boolean.json"
+}

source/resource/ui/elements/properties/use_child_anchors.json

@@ -0,0 +1,5 @@
+{
+  "title": "Use Child Anchors",
+  "description": "If true, this element will stack its controlled element in respect to their anchor values.",
+  "$ref": "../../general/boolean.json"
+}