Add an explicit way to set which page is the starting page

[mvladic]

We should probably have an explicit way to mark a page as starting page instead of moving to the top of the Pages list. This is especially problematic if user sorts its pages alphabetically and forgets about it.

• Button "Set as Start Page" in page properties
• Context menu: "Set as Start Page"
• "Start Page" tag in Pages list show for the starting page.