IPC-2581 exporter should export optional layer characteristics

No board stack up information is included in the IPC-2581 export XML.

Layer specifications could be added to the ; section of the XML and Layer thicknesses could be added to the ; section (must be after the definitions). The information seems mostly to be present in the PCB editor board setup window. Importing the XML into Ansys SiWave fails without stack-up data which is a pain. I have manually modified an exported .xml file to include the stackup data and included the relevant excerpt below as an example. Kicad produces the sections already already but I kept it in the except to show where the data needs to be - I hope this is of some help. My manual workaround can be done in Notepad++ but it would be really helpful for it to be automatic.

KiCad Version

Application: KiCad PCB Editor x64 on x64

Version: 8.0.0-rc1-67-g7b24167398, release build

Libraries:
	wxWidgets 3.2.4
	FreeType 2.12.1
	HarfBuzz 8.3.0
	FontConfig 2.14.2

Platform: Windows 10 (build 19045), 64-bit edition, 64 bit, Little endian, wxMSW

	wxWidgets: 3.2.4 (wchar_t,wx containers)
	Boost: 1.83.0
	OCC: 7.7.1
	Curl: 8.4.0-DEV
	ngspice: 42
	Compiler: Visual C++ 1936 without C++ ABI

Build settings: