PcbNew: don't clear highlight on Esc; change highlight tool to toggle button

Description

Pressing tilde on a net in the schematic editor highlights that net.

Pressing tilde on another net switches highlighting to that other net.

Pressing tilde on empty space clears all net highlighting.

Pressing escape in the PCB editor clears all net highlighting but not as of yet for the schematic editor.

Steps to reproduce

1. Highlight net in schematic editor
2. Press escape and observe that net highlighting is not cleared.

KiCad Version

Application: KiCad Schematic Editor

Version: (5.99.0-10362-gce3a84f579), release build

Libraries:
	wxWidgets 3.0.5
	libcurl/7.71.0 OpenSSL/1.1.1g (Schannel) zlib/1.2.11 brotli/1.0.7 libidn2/2.3.0 libpsl/0.21.0 (+libidn2/2.3.0) libssh2/1.9.0 nghttp2/1.41.0

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

Build Info:
	Date: Apr 20 2021 11:02:54
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8)
	Boost: 1.73.0
	OCE: 6.9.1
	Curl: 7.71.0
	ngspice: 34
	Compiler: GCC 10.2.0 with C++ ABI 1014

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
	KICAD_USE_OCE=ON
	KICAD_SPICE=ON