Add rounding (fillets) to shapes

Description

The new nice and useful rectangle graphic shape could have parameter for rounded corners (fillets) like rounded rectangle pads have.

To be honest I can't find much use for it in footprints - courtyard, fab layer or silk features won't usually have rounded corners. Edge.Cuts would be the layer where it would be useful. Many board outlines are simple rectangles with rounded corners. It would also be convenient for inner slots. Creating the corresponding shape and especially changing it again and again is tedious with four straight lines and four arcs.

A prerequisite is of course having rectangles in the board editor.

KiCad Version

Application: Pcbnew
Version: (5.99.0-2062-g224b41e8b), release build
Libraries:
	wxWidgets 3.0.4
	libcurl/7.66.0 OpenSSL/1.1.1d (Schannel) zlib/1.2.11 brotli/1.0.7 libidn2/2.2.0 libpsl/0.21.0 (+libidn2/2.1.1) nghttp2/1.39.2
Platform: Windows 8 (build 9200), 64-bit edition, 64 bit, Little endian, wxMSW
Build Info:
	Build date: Jun 28 2020 21:09:37
	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8)
	Boost: 1.71.0
	OCE: 6.9.1
	Curl: 7.66.0
	Compiler: GCC 9.2.0 with C++ ABI 1013

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