Make MPO tweak a three-state control (#4897)

* refactor: make MPO tweak a three-state control

* docs: explain MPO tweak states

* docs: address code review comments

* refactor: make Multiplane Overlay config-driven

functions/private/Get-WinUtilRegistryComboValue.ps1

@@ -0,0 +1,24 @@
+function Get-WinUtilRegistryComboValue {
+    <#
+    .SYNOPSIS
+        Reads one registry value for a registry-backed combo-box state.
+
+    .PARAMETER Setting
+        The registry setting from the combo-box configuration.
+    #>
+    param(
+        [Parameter(Mandatory)]
+        $Setting
+    )
+
+    try {
+        $item = Get-ItemProperty -Path $Setting.Path -Name $Setting.Name -ErrorAction Stop
+        $property = $item.PSObject.Properties[$Setting.Name]
+        return [pscustomobject]@{ Exists = $null -ne $property; Value = $property.Value }
+    } catch [System.Management.Automation.PSArgumentException] {
+        # The registry provider uses PSArgumentException when a named value is absent.
+        return [pscustomobject]@{ Exists = $false; Value = $null }
+    } catch [System.Management.Automation.ItemNotFoundException] {
+        return [pscustomobject]@{ Exists = $false; Value = $null }
+    }
+}