Pcbnew: Board Setup - Netclass Editor allows wrong inputs plus some minor issues

Description

2020-08-27_17-30-09 Changing net class names of a net results in some minor bugs/UX issues:

1. Clicking on a net class removes the value
2. Entering some invalid net class is accepted
3. Autocomplete for the net class names would be nice
4. After entering a net class and clicking on another net, the net class from before is automatically applied. Not sure if this is a bug or intentional.

Steps to reproduce

1. Open Pcbnew->Board Setup->Net Classes
2. Click on "Net Class" element on the right
3. Observe that the value from before is deleted
4. Enter some random name that is not present as as net class
5. Click on another element and observe that the net class name from before is copied
6. Click "Ok" and reopen the window
7. Old net class values are restored

KiCad Version

Application: Pcbnew

Version: (5.99.0-3029-ga08c5c67e), debug build

Libraries:
	wxWidgets 3.0.4
	libcurl/7.68.0 OpenSSL/1.1.1f zlib/1.2.11 brotli/1.0.7 libidn2/2.2.0 libpsl/0.21.0 (+libidn2/2.2.0) libssh/0.9.3/openssl/zlib nghttp2/1.40.0 librtmp/2.3

Platform: Linux 5.4.0-42-generic x86_64, 64 bit, Little endian, wxGTK

Build Info:
	Date: Aug 27 2020 16:19:59
	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.71.0
	OCE: 6.9.1
	Curl: 7.68.0
	ngspice: 31
	Compiler: GCC 9.3.0 with C++ ABI 1013

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