pcbnew appearance manager: Focus inside blank space in appearance manager disables keyboard shortcuts

Description

Keyboard shortcuts stop working when clicked into blank area in appearance widget. Similar to #5225 (closed).

Steps to reproduce

1. Click inside blank area in the appearance widget (for example to the blank space between list of layers and Layer Display Options on the Layers tab).
2. Keyboard shortcuts don't work until mouse leaves appearance widget.

KiCad Version

Application: Pcbnew

Version: (5.99.0-2845-ga029feb02-dirty), release build

Libraries:
	wxWidgets 3.0.4
	libcurl/7.67.0 OpenSSL/1.1.1d (Schannel) zlib/1.2.11 brotli/1.0.7 libidn2/2.3.0 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: Aug 18 2020 20:27:34
	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8)
	Boost: 1.72.0
	OCE: 6.9.1
	Curl: 7.67.0
	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