Saves about 473 byte of progmem and 28 bytes of RAM.

Apply `ENABLED` / `DISABLED` macros to files needing only a small
number of changes.

Replaced calculation to centre bitmap with fixed values.
Saved 20 bytes.

Here comes a compile-time operation to match the extruder symbols on the
info screen to the set number of extruders in configuration.h.
When only one extruder is selected, the "1" on the symbol will not be
displayed.

Diffed and merged, preserving my updates

1. Marlin welcome screen
2. Status screen symbols 3 Extruders, heated bed, animated fan logo 1
3. Status screen symbols 3 Extruders, heated bed, animated fan logo 2