Support inverted text "boxes" with customizable decorations

Description

We should support inverting text objects in PcbNew / footprint editor. This means creating a polygon "text box" with the letter shapes punched out. This text box shape should support some decoration on the end caps such as rounded or angled caps.

This allows (among other things) more pleasant silkscreen annotations of pins: https://twitter.com/arturo182/status/1348415792254169089

Split off from #4122

KiCad Version

Application: Pcbnew

Version: (5.99.0-8184-g46c4727783), debug build

Libraries:
	wxWidgets 3.1.4
	libcurl/7.71.1-DEV Schannel zlib/1.2.11

Platform: Windows 10 (build 19041), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Jan  5 2021 21:03:10
	wxWidgets: 3.1.4 (wchar_t,STL containers)
	Boost: 1.73.0
	Curl: 7.71.1-DEV
	ngspice: 32
	Compiler: Visual C++ 1928 without C++ ABI

Build settings:
	KICAD_SCRIPTING=OFF
	KICAD_SCRIPTING_MODULES=OFF
	KICAD_SCRIPTING_PYTHON3=OFF
	KICAD_SCRIPTING_WXPYTHON=OFF
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
	KICAD_SCRIPTING_ACTION_MENU=OFF
	KICAD_SPICE=ON
	KICAD_STDLIB_DEBUG=OFF
	KICAD_STDLIB_LIGHT_DEBUG=OFF
	KICAD_SANITIZE=OFF