Helper pane - display information when empty

[BlazZupan]

When help is on (by default), it shows widget help, but only on widget hoover or when the widget is selected. Else, the window is currently empty.

Instead of an empty window, show "Select a widget to show its description. Or see workflow examples, youtube tutorials, or open a welcome screen." Make sure that derivatives of Orange (like scOrange) open the right windows or go to the right youtube channel.

In addition, change some item names in the help menu.

• Tutorials -> YouTube Tutorials
• Examples -> Workflow Examples