Refactoring: move all code files into a src directory

src/UI/Base/If.svelte

@@ -0,0 +1,24 @@
+<script lang="ts">
+  import { UIEventSource } from "../../Logic/UIEventSource"
+  import { onDestroy } from "svelte"
+
+  /**
+   * For some stupid reason, it is very hard to let {#if} work together with UIEventSources, so we wrap then here
+   */
+  export let condition: UIEventSource<boolean>
+  let _c = condition.data
+  onDestroy(
+    condition.addCallback((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`! */
+      _c = c
+      return false
+    })
+  )
+</script>
+
+{#if _c}
+  <slot />
+{:else}
+  <slot name="else" />
+{/if}