Refactoring: move all code files into a src directory

src/UI/Base/IfHidden.svelte

@@ -0,0 +1,34 @@
+<script lang="ts">
+  import { UIEventSource } from "../../Logic/UIEventSource"
+  import { onDestroy } from "svelte"
+
+  /**
+   * Functions as 'If', but uses 'display:hidden' instead.
+   */
+  export let condition: UIEventSource<boolean>
+  let _c = condition.data
+  let hasBeenShownPositive = false
+  let hasBeenShownNegative = false
+  onDestroy(
+    condition.addCallbackAndRun((c) => {
+      /* Do _not_ abbreviate this as `.addCallback(c => _c = c)`. This is the same as writing `.addCallback(c => {return _c = c})`, 
+        which will _unregister_ the callback if `c = true`! */
+      hasBeenShownPositive = hasBeenShownPositive || c
+      hasBeenShownNegative = hasBeenShownNegative || !c
+      _c = c
+      return false
+    })
+  )
+</script>
+
+{#if hasBeenShownPositive}
+  <span class={_c ? "" : "hidden"}>
+    <slot />
+  </span>
+{/if}
+
+{#if hasBeenShownNegative}
+  <span class={_c ? "hidden" : ""}>
+    <slot name="else" />
+  </span>
+{/if}