Altium board importer, bugfixes #4 (and new features)

I set it to WIP because this will grow to a bigger MR which should fix a few bigger issues, and thus will require more testing.

Goals:

• improve Pad handling
• improve Text handling
• add 3D-Model import support

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Bugfixes:

• Add missing SetWidth( 0 ) in Fills
• Apply correct text rotation

Improvements:

• Support Pads on non-copper layers (we need to simulate them using drawings)
• Support hole offset
• Step import
• assign layer type