PNS and Place Via: active tool overrides user decision to change grid size

Description

The user decision to change the grid size via the dropdown is overwritten with the previous value when PNS or Place Via are active. ALT+1, ALT+2, N and SHIFT+N shortcuts to modify the grid are ineffective.

This seems like an artificial restriction and other tools don't put up a fight.

This issue proposes enabling grid size updates for PNS and Place Via.

Steps to reproduce

1. select PNS
2. change grid size

KiCad Version

Version: (5.99.0-3688-g85444aadd), release build, Oct 1 2020 09:07:32

Application: Pcbnew

Version: (5.99.0-3688-g85444aadd), 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 8 (build 9200), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Oct  1 2020 09:07:32
	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: 32
	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
	BUILD_GITHUB_PLUGIN=ON
	KICAD_USE_OCE=ON
	KICAD_SPICE=ON