display: Replace hard-coded display with new config based display

Introduce a new config based system for specifying the on-screen
contents of an lcd screen.  The default screen configuration (found in
klippy/extras/display/display.cfg) is the same as the previous
hard-coded display, so this should not change behavior for existing
users.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/display/icons.py

@@ -141,6 +141,6 @@ feedrate_icon = [
 Icons16x16 = {
     'extruder': extruder_icon,
     'bed': bed_icon, 'bed_heat1': bed_heat1_icon, 'bed_heat2': bed_heat2_icon,
-    'fan1': fan1_icon, 'fan2': fan2_icon,
+    'fan': fan1_icon, 'fan1': fan1_icon, 'fan2': fan2_icon,
     'feedrate': feedrate_icon,
 }