v1.2.1 Retraction Length 0 disables Firmware Retraction

[mitchese]

Choosing the option "Use firmware retraction" in Printer/General only generates G10/G11 if the option "Retraction Length" in Printer/Extruder x is nonzero (not disabled).

In the 1.2.1 Version, selecting "Use firmware retraction" in Printer/General disables the box to set the retraction length (in my opinion, correct behaviour). If this field was previously set to zero, it is now locked at zero and not possible to change until firmware retraction option is unselected.

Steps to re-create:

1. Set Retraction Length to zero
2. Set to use firmware retraction
3. Generate G-Code -- no G10/G11 anywhere in the output
4. Try to change Retraction Length to 1 -- input is disabled
5. Disable firmware retraction
6. Change retraction length to a nonzero value (1)
7. Re-enable firmware retraction
8. Generate G-code: now correct with G10/G11

[alranel]

Thank you, I fixed this and added a regression test.