Left side menu should be toggleable

Variant 1 (preferable for first cut)

• arrow icon on the bottom of the manu hides the left side menu
  providing only the possibility to restore it.

Later on, the v1 behavior can be enhanced - NOTE: this is not a subject for this ticket

• when menu is toggled, only the root menu icons are displayed

• click to the root menu icon goes to the default page specific for each root menu
  There is some uncertainty currently how this may look&feel&work, the preference is
  to postpone this work once the menu and its options are well known & defined.