Polish main sidebar

This MR polish the main side bar to:

• Enforce the same size for every buttons
• Use same horizontal align for every buttons
• Center shortcuts
• Allow to collapse/extend the sidebar (mostly for new users so what they don't have too much to think about that strange glyphs)

Changes:

• The main side bar use vertical flex display .sidebar-group separated by .sidebar-expand
• A SidebarButton is provided for each icons
• The last button allow to collapse/extend the sidebar

I am fine with the look and feel, i am still not total convinced by the labels Samples/Tasks/Collections, it have to be short and meaningful. This could be part of another MR cause i would like to change some icons at the same time.