Feature/multiscreen notifications

• Fix flags for DesktopPopup
• Merge notification creation and show macros
• Store height offset for each screens (key: screen name)
• Create a Visual Root Window for each notifications and bind them to a Screen
• Put the Visual Root Window behind its notification to get visually only one entity
• Propagate events between visual objects in order to use only one
• Add open/close events
• Add missing anchors
• Avoid using window as a keyword