Overhaul of the subnavigation and further smoother icons

This release is compatible with My Little Forum 2.4.20.

Changes

• rework of a further few icons with unobstrusive gradients
• a new icon with a square with rounded corners (currently not in use)
• new icons for navigating to next or previous $anything in active and inactive states (previous: active, inactive and next: active and inactive)
• rework of the HTML-structure and CSS-rulesets of the subnavigation below the page header
  • links in the user menu and page actions take the whole width of the viewport in narrow viewports (like smartphones in portrait mode)
  • the links to navigate to the previous and next pages are shown all the time (when the forum has more than one page in the main view) but are inactive when they are unnecessary

Fix

• in conjunction with the rework of the subnavigation I found the multiple use of the ID subnav-2, renamed two occurences of the ID