Flag and Favicon visors enhancement

[amrrizq]

The following should be done for enhancing favicon and flag visors:

1. Add an expressive icon indicating unavailability of flag for a node.
2. The favicon is cached if it exists. But it's not cached if it doesn't exist. So cache it.
3. In node settings, under shape category, the following options should be re-implemented as follows:
   • Visor-based
   • Visor-based inside geometric shape
   • Geometric shape with solid color
     If the last option is selected, the visors (currently flag visor and favicon visor) should be disabled