AppExplorer: Refactored dashboard tab icon for consistent display

Now uses a generalized CSS approach and will work with normal 16x16
icons. This is handled via new CSS class 'ra-tab-icon-only-16px' (see
in-code comments).

Converted the home icon to a normal 16x16 image (home.png) in the icon
assets ('ra-icon-home').

This will hopefully solve the inconsistent display issues that have
continued to crop up across different browsers (as well as being a much
cleaner and more generalized API to support use elsewhere)