Add icons and beta marker for tab designer

We need

• a container icon for "Tabs" element in the tree,
• an item icon for a given "Tab" instance in the tree and on its main page,
• a "beta feature" icon visible on tree and on each of the tab feature editors, with a text explaining that feature is new.
• a "new feature" icon to be displayed only on tree
• a design improvement for warnings and error stickers in the tree
• icons for content view while we're at it