[Feature Request] Add custom start G-Code in Filament Settings

[tcm0116]

I've been playing around with Marlin's Linear Advance feature lately, and one thing that's been a little annoying is that if I change filaments, I have to modify the Start G-Code in the Printer Settings to provide the proper K value in the M905 (now M900 in very recent versions of Marlin). It occurred to me that it would be very useful feature to have a custom start G-Code block in the filament settings that would let me do things on a per-filament basis as opposed to having to modify the printer-level settings.

[fiveangle]

Agree++

[bubnikv]

Implemented with ad3be1a

Based on the implementation by @lordofhyphens
slic3r/Slic3r@19eea19