Various infill improvements

[Noisyfox]

1. Fix issue that sparse infill threshold no longer working
2. Turn all internal sparse infill into solid infill if infill density is 100%
3. Allow combining solid infill when sparse infill density is 100%
4. Move internal solid infill pattern settings into the Infill section

This should fix #2645

[SoftFever]

The sparse infill threshold in 1.8 works fine for me.
In which situation it didn't work?

Left with threshold set to 15, right 0

sparse-infill-threshold.zip

[Noisyfox]

Yeah there are some extreme rare cases that could reproduce the problem, such as the model from #2645:

As shown in the screenshot, the red area is no where close to 10000 mm2 yet it is still sparse infill.

[SoftFever]

Looks good