Fix Infill setting visibility (and improve descr.)

[Sophist-UK]

Description

This fixes the Infill sub-settings visibility when you have no infill.

1. Add enabled lines where needed to make infill settings disappear when Density is 0
2. Amend enabled lines to hide when Line Distance is 0
3. Amend description of Line Distance to give more details about the impact of overriding the calculated value.
4. Merge multiplier = 2 options when calculating Line Distance.
5. Indent Support Angle.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

I have made this same update in my installation of Cura 5.6.0 and tested that the changes work.

Test Configuration:

• Windows 10
• Cura 5.6.0
• File: C:\Program Files\UltiMaker Cura 5.6.0\share\cura\resources\definitions\fdmprinter.def.json

Checklist:

• My code follows the style guidelines of this project as described in UltiMaker Meta and Cura QML best practices
• I have read the Contribution guide
• I have commented my code, particularly in hard-to-understand areas
• I have uploaded any files required to test this change