Symbol editor: pin grid should allow filtering (e.g. by current unit)

Broken out from #9346. Related: #9383 (closed)

Description

I have a component with many units (A though BB). I often want to use the pin view to work on just one unit (i.e. when assigning X/Y coordinates).

• Current behaviour: pin grid shows all pins in the component
• Desired behaviour: allow grid to be filtered so it can show either "all units" or just pins in the currently opened unit.

KiCad Version

Application: KiCad Symbol Editor

Version: 5.99.0-unknown-57c7d663b0~140~ubuntu20.04.1, release 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 5.9.0-2-amd64 x86_64, 64 bit, Little endian, wxGTK, , 

Build Info:
	Date: Oct  7 2021 18:24:31
	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.71.0
	OCC: 7.5.2
	Curl: 7.68.0
	ngspice: 31
	Compiler: GCC 9.3.0 with C++ ABI 1013

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON