Add "open plugin folder" to plugin menu

Description

the plugin folder is currently a bit tedious to find or get to. adding a link to open the plugins folder in the external plugins menu would help a lot.

aka:

tools -> external plugins -> open plugins folder

KiCad Version

not present in 5.99.0-2402-g2af6d01f

Application: Pcbnew

Version: (5.99.0-2402-g2af6d01fd), release build

Libraries:
	wxWidgets 3.0.4
	libcurl/7.66.0 OpenSSL/1.1.1d (Schannel) zlib/1.2.11 brotli/1.0.7 libidn2/2.2.0 libpsl/0.21.0 (+libidn2/2.1.1) nghttp2/1.39.2

Platform: Windows 8 (build 9200), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Jul 25 2020 23:47:08
	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8)
	Boost: 1.71.0
	OCE: 6.9.1
	Curl: 7.66.0
	ngspice: 31
	Compiler: GCC 9.2.0 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