Grid thickness default is (probably) invisible on 4k screens

Description

As a new user, the dots did not show up on my 4k 21.5" iMac and my first thought was that there was something wrong. But then: a pixel turned out be too small on my screen. I suggest that the grid thickness default is set in steps, depending on the screen pixel width. I ended up with 3.0 pixels. (I don't think it needs to be set depending on age, even if that probably would have been a good idea (pun intended)). I have just now also filed another matter about grid thickness: Issue 11162: Grid thickness does not survive crash.

Maybe grid thickness and dot thickness should have been separate parameters?

Steps to reproduce

Experiment with grid thickness over screen sizes
Imagine you're 70+ and wear glasses (no pun) as an argument to actually do as I suggest

KiCad Version

Application: KiCad Schematic Editor

Version: (6.0.2-0), release build

Libraries:
	wxWidgets 3.1.5
	libcurl/7.54.0 LibreSSL/2.6.5 zlib/1.2.11 nghttp2/1.24.1

Platform: macOS Mojave Version 10.14.6 (Build 18G9323), 64 bit, Little endian, wxMac

Build Info:
	Date: Feb 11 2022 19:53:03
	wxWidgets: 3.1.5 (wchar_t,wx containers)
	Boost: 1.76.0
	OCC: 7.6.0
	Curl: 7.64.1
	ngspice: 35
	Compiler: Clang 12.0.0 with C++ ABI 1002

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON