Pcbnew : drawing arcs with start / end points first.

Description

When drawing an arc in Pcbnew, we first select the center of the arc, then two points. Sometimes, we know where we want the arc to start and to end, but the radius is not the main concern.

Therefore, I think having an alternate drawing method for drawing arcs would be great. Using this method, the user would first select two points that define the arc start/ end points, A and B, and then would change the curvature of the arc, by moving the center of the arc, C.

I think this would be helpful in some cases, including if a trace needs to follow a complex board outline. ( IFAIK, we can't snap the end point to a track)

In this image, the position of C is not that easy to guess, so the arc can meet both A and B correctly:

Steps to reproduce

KiCad Version

Application: Pcbnew

Version: 5.99.0-unknown-93e3268~102~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 5.4.0-42-generic x86_64, 64 bit, Little endian, wxGTK

Build Info:
	Date: Aug  1 2020 10:17:29
	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.71.0
	OCE: 6.9.1
	Curl: 7.68.0
	ngspice: 31
	Compiler: GCC 9.3.0 with C++ ABI 1013

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