Board Setup: changes to physical stackup layer count are reverted when editing other properties

Description

Steps to reproduce

1.2021-07-16_14-10-30 2.

KiCad Version

Application: KiCad PCB Editor (64-bit)

Version: (5.99.0-11439-g7ff9806386), release build

Libraries:
	wxWidgets 3.1.5
	libcurl/7.74.0-DEV Schannel zlib/1.2.11

Platform: Windows 10 (build 19042), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Jul 16 2021 09:00:25
	wxWidgets: 3.1.5 (wchar_t,STL containers)
	Boost: 1.75.0
	OCC: 7.5.0
	Curl: 7.74.0-DEV
	ngspice: 34
	Compiler: Visual C++ 1928 without C++ ABI

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON