#ttp 337318 EDITOR: CRASH: Changing viewport layout to one pane and entering immersive mode causes a crash
reviewer Thomas.Sarkanen
[CL 2096929 by Andrew Copland in Main branch]
#branch UE4
#proj Editor.LevelEditor
#summary Added 1x1 viewport layout and tidied the margins in the layout menu
#add Added FLevelViewportLayout::bIsMaximizeSupported and public getter IsMaximizeSupported(). This is set on the new one-pane layout to block the maximize command/button.
#add LevelViewportConfigurationNames::OnePane - new named layout config.
#add FLevelViewportCommands::ViewportConfig_OnePane - new layout command.
#change FLevelViewportLayout::InitCommonLayoutFromString() - will never start a layout maximised if bIsMaximizeSupported is false.
#change FLevelViewportLayout::SaveCommonLayoutString() - will never save a layout maximised if bIsMaximizeSupported is false.
#change FLevelViewportLayout::MaximizeViewport() - checks that bIsMaximizeSupported is true as it should never be called on a layout that doesn't support it.
#add SlateEditorStyle - added brushes for one pane option in the viewport layout menu.
#add SLevelViewportToolBar::GenerateViewportConfigsMenu() - new section for one pane layouts with the one pane button in it.
#change SLevelViewportToolBar::GenerateViewportConfigsMenu() - fixed the layout margins by setting the label visibility to collapsed instead of passing an empty string as the label.
#add Binding new ViewportConfig_OnePane command SLevelViewport::BindViewCommands to OnSetViewportConfiguration().
#change SLevelViewport::GetMaximizeToggleVisibility() & OnToggleMaximize - check IsMaximizeSupported() on the parent layout and always collapses the maximize button when it isn't supported.
#add FLevelViewportTabContent::ConstructViewportLayoutByTypeName() - added LevelViewportConfigurationNames::OnePane.
#add New icon image for one pane button in the menu - ViewportLayout_OnePane.png
#add New layout class FLevelViewportLayoutOnePane - based on other layouts but simpler because it only contains one pane and therefore no splitters.
reviewedby Thomas.Sarkanen, Max.Preussner
[CL 2048729 by Chris Wood in Main branch]
