Added M73 and M73 specific for Prusa support

[fietser28]

During USB printing from Octoprint, the % and time stay empty on the display of my Prusa i3 MK3.

I've added configurable support for the M73 (progress) gcode command. This uses the progress field (%) or bar (other printers) of marlin firmware that have this option enabled (e.g. Pursa i3 MK3).

I've also added an option to send Prusa specific (firmware >= 3.3.0) commands that show the remaining time on the display (see picture).

Documentation is also updated to describe the new settings.

Other separate M73 plugins did not update properly and don't have support for the remaining time of the Prusa firmware, so I decided to add this to detailedprogress myself.

Purse is going to add these commands to slic3r, but that gives a static estimate instead of the dynamic options/plugins we now see from Octoprint.

If you like this addition please merge into the main branch.

[zimmermannb]

This is perfect, exactly what I was looking for.

@dattas Would it be possible to merge this into master, to have it in the next release?

[tpmullan]

I have added this to my fork, and have the changes available here: https://github.com/tpmullan/OctoPrint-DetailedProgress