Add comprehensive test suite, fix minor issues, rename function and file names to match approved verbs (#708)

Scripts/Threading/Invoke-DoEvents.ps1

@@ -0,0 +1,16 @@
+# Processes all pending WPF window messages (input, render, etc.) to keep the UI responsive
+# during long-running operations on the UI thread. Equivalent to Application.Invoke-DoEvents().
+function Invoke-DoEvents {
+    if (-not $script:GuiWindow) { return }
+    $frame = [System.Windows.Threading.DispatcherFrame]::new()
+    $null = [System.Windows.Threading.Dispatcher]::CurrentDispatcher.BeginInvoke(
+        [System.Windows.Threading.DispatcherPriority]::Background,
+        [System.Windows.Threading.DispatcherOperationCallback]{
+            param($f)
+            $f.Continue = $false
+            return $null
+        },
+        $frame
+    )
+    $null = [System.Windows.Threading.Dispatcher]::PushFrame($frame)
+}