UE-191200 - "Add UE support for [at]editable fields of optional types"

Add's support for `[at]editable` on optionals:

`FOptionalProperty` changes:
- Added custom `__INIT__` logic for `FOptionalProperty::ExportText` which previously could output no text as the optional value was initialized but returned an empty string as its export text (ie: empty arrays, maps, sets, etc). This caused the future text imports to be incorrect.

`FPropertyNode` changes:
- Added `OptionalValueNode` as private member similar in idea to existing `PropertyKeyNode`. It stores the generated `FPropertyNode` for a FOptionalValue's Value when it is set.
- Added helper function `GetOrCreateOptionalValueNode` which either returns the existing `OptionalValueNode` OR creates one if appropriate.
    - Note: This is where we bind a lambda to our Optional's Value FPropertyNode's `RebuildChildren` event which is how we update/rebuild when needed. In places where we would update/rebuild we instead call our Value to do so instead if set and then update after that has been done via this callback.

Added new widget `SPropertyEditorOptional`
- This either:
    - shows a dropdown of "Set all" or "Unset all" if we have multiple values selected
    - generates the editor for whatever our set value is if we have a single value (See `GetSingleReadAddress`)
    - shows a 'set value' button if we are not set

#JIRA UE-191200
#rb karen.jirak
#rb kurtis.schmidt

[CL 28253719 by jared cotton in ue5-main branch]

Engine/Source/Editor/PropertyEditor/Private/PropertyCustomizationHelpers.cpp

@@ -175,6 +175,37 @@ namespace PropertyCustomizationHelpers
 			.IsFocusable( false );
 	}
 
+	TSharedRef<SWidget> MakeSetOptionalButton(FSimpleDelegate OnSetOptionalClicked, TAttribute<FText> OptionalToolTipText, TAttribute<bool> IsEnabled)
+	{
+		// Custom widget for this button as it has no image and should fill a larger space
+		return SNew(SBox)
+			.HAlign(HAlign_Left)
+			.VAlign(VAlign_Center)
+			.ToolTipText(OptionalToolTipText.Get().IsEmpty() ? LOCTEXT("SetButtonToolTipText", "Set Optional to default value.") : OptionalToolTipText)
+			[
+				SNew(SButton)
+				.ButtonStyle(FAppStyle::Get(), "Button")
+				.OnClicked_Lambda([OnSetOptionalClicked](){
+					OnSetOptionalClicked.ExecuteIfBound();
+					return FReply::Handled();
+				})
+				.Text(LOCTEXT("SetButtonText", "Set to Value"))
+				.ContentPadding(0)
+				.IsFocusable(false)
+			];
+	}
+
+	TSharedRef<SWidget> MakeClearOptionalButton(FSimpleDelegate OnClearOptionalClicked, TAttribute<FText> OptionalToolTipText, TAttribute<bool> IsEnabled)
+	{
+		return
+			SNew(SPropertyEditorButton)
+			.Text(OptionalToolTipText.Get().IsEmpty() ? LOCTEXT("ClearButtonToolTipText", "Clear Optional") : OptionalToolTipText)
+			.Image(FAppStyle::Get().GetBrush("Icons.X"))
+			.OnClickAction(OnClearOptionalClicked)
+			.IsEnabled(IsEnabled)
+			.IsFocusable(false);
+	}
+
 	FText GetVisibilityDisplay(TAttribute<bool> bEnabled)
 	{
 		return bEnabled.Get() ? FEditorFontGlyphs::Eye : FEditorFontGlyphs::Eye_Slash;