EEschema: Add support for displaying unmounted components

Description

It would be good to be able to mark components that will not be mounted in the design, I am enclosing an example of how this illustration is solved in Altium designer.

KiCad Version

Application: Eeschema

Version: 5.99.0-unknown-f97c50b~102~ubuntu18.04.1, release build

Libraries:
	wxWidgets 3.0.4

Platform: Linux 5.3.0-62-generic x86_64, 64 bit, Little endian, wxGTK

	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.22
	Boost: 1.65.1
	OCE: 6.9.1
	Curl: 7.58.0
	ngspice: 32+
	Compiler: GCC 7.5.0 with C++ ABI 1011

Build settings:
	KICAD_SCRIPTING=ON
	KICAD_SCRIPTING_MODULES=ON
	KICAD_SCRIPTING_PYTHON3=ON
	KICAD_SCRIPTING_WXPYTHON=ON
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
	KICAD_SCRIPTING_ACTION_MENU=ON
	BUILD_GITHUB_PLUGIN=ON
	KICAD_USE_OCE=ON
	KICAD_SPICE=ON