Add GCODE macros to printers panel#178

I want to be able to send custom GCODE from the printer panel to send gcode specifically to different groups of printers from this page from the drop-down.

I think to avoid any concerns of invalid or bad gcode being sent to the printers doing it at a group level should be fine. Or perhaps another enchantment to add tags to gcode macros so it can only be sent to matching printers. This would be probably important if users have different printers in each of there groups in-case they have different printers on a rack.

Another perhaps I’ll make this as another request but It would be nice to make a master gcode macro aka unload filament, then have the ability to define a tag and each sub-macro for each different model. In-case the macros are different across machines.

This is probably a low-priority ticket, getting by doing this from the GCODE menu is fine for now, but would be nice to have it in 1 spot.