Color settings resets opacity if opacity is 0

Description

If you change the opacity in EESchema's color proferences to 0% then when you reopen the color picker it gets reset to 100%, but doesn't get reset when the opacity is above 0%.

Edit: The same happens in PCBNew's color picker.

Steps to reproduce

1. Go to Color preferences in EEschema.
2. Select a color to change, set the opacity to 0%.
3. Reopen the same color setting, and see that the opacity has been changed to 100%.

KiCad Version

marked dirty because of #5013 
Application: Eeschema

Version: (5.99.0-2499-g6e4d4401a-dirty), debug build

Libraries:
	wxWidgets 3.0.4
	libcurl/7.69.1 OpenSSL/1.1.1g-fips zlib/1.2.11 brotli/1.0.7 libidn2/2.3.0 libpsl/0.21.0 (+libidn2/2.3.0) libssh/0.9.4/openssl/zlib nghttp2/1.41.0

Platform: Linux 5.7.10-201.fc32.x86_64 x86_64, 64 bit, Little endian, wxGTK

Build Info:
	Date: Jul 31 2020 16:00:57
	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.69.0
	OCC: 7.4.0
	Curl: 7.69.1
	ngspice: 32
	Compiler: GCC 10.2.1 with C++ ABI 1014

Build settings:
	KICAD_SCRIPTING=ON
	KICAD_SCRIPTING_MODULES=ON
	KICAD_SCRIPTING_PYTHON3=ON
	KICAD_SCRIPTING_WXPYTHON=ON
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
	KICAD_SCRIPTING_ACTION_MENU=ON
	BUILD_GITHUB_PLUGIN=ON
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON
	KICAD_STDLIB_DEBUG=OFF
	KICAD_STDLIB_LIGHT_DEBUG=OFF
	KICAD_SANITIZE=OFF