[2.0.x] Show manual extrusion moves as relative in LCD

[marcio-ao]

Summary:

• Implements FR#11724, Make manual E moves relative (esp. multi-extruder)
• Old behavior can be restored by commenting out MANUAL_E_MOVES_RELATIVE

Detailed Description:

Suppose the user wants to extrude 50mm from E1 and 10mm from E2 at powerup. Currently, this is what they have to do:

• Pre-heat nozzles
• Go to Movement > Move Axis > Extruder 1
• Turn the knob until the position goes from 0mm to 50mm (50 mm will be extruded from E1)
• Go to Movement > Move Axis > Extruder 2
• Turn the knob until the position goes from 50mm to 60mm (10 mm will be extruded from E2)

To an end-user this makes no sense and isn't particularly helpful if they want to extrude or retract a particular amount of filament without having to do math. For the users, it would make more sense if it worked like this:

• Pre-heat nozzles
• Go to Movement > Move Axis > Extruder 1
• Turn the knob until the position goes from 0mm to 50mm (50 mm will be extruded from E1)
• Go to Movement > Move Axis > Extruder 2
• Turn the knob until the position goes from 0mm to 10mm (10 mm will be extruded from E2)

This PR enables this functionality by storing the current value of the E axis when the user enters the "Extruder " screen and subtracting that out from the value that is shown in the adjuster. That way, the adjuster will always begin at 0 mm and will represent a relative offset, rather than an absolute position.

If someone prefers to see absolute positions, this can be restored by commenting out MANUAL_E_MOVES_RELATIVE (I suggest this feature be left enabled by default in the config as it improves usability for most users).