Added option to independently set z axis travel speed

[martinbudden]

Added an option travel_speed_z to independently set the Z axis travel speed. I've set the default value to 12 which is the default value of machine_max_feedrate_z.

This addresses issues #2945 , #6365 and #3995

Currently the same travel speed is used for x, y and z, which can means the z travel speed can be high which causes problems such as Z-banding and even can cause the Z-motor to skip (as reported in the issues).

In theory the z travel speed should be limited by the printer firmware using M203 and the value machine_max_feedrate_z, however not all printer firmware supports this.

This is my first PR for PrusaSlicer, so I'd appreciate it if someone could check if I've made any obvious mistakes.

[martinbudden]

Update - well it turns out I did miss something obvious - I've not updated the test code to reflect this change, so I'll make an update to this once I've figured out how to run the test code.

[lukasmatena]

Hi. I have merged your commit (bd6badf) and added a little amendment (4657e0d). The point of it is to make sure the default value does not break existing profiles. There are printers with different kinematics than i3 and we were worried we might break something that worked before. I'll be glad if you review it.

Thanks for your contribution.
Closing.