Backspace should undo last segment for graphical line

Description

Implement "Undo last segment" hotkey, in the same manner as "backpace" key does while routing.

0109J

Steps to reproduce

1. Draw multisegment line in bottom silk layer, hit backspace -> Nothing happens.
2. It is expected for last segment to be deleted after hitting backspace.

KiCad Version

Application: KiCad PCB Editor (64-bit)

Version: (6.0.0-rc1-452-gf844f5a80c), release build

Libraries:
	wxWidgets 3.1.5
	libcurl/7.78.0-DEV Schannel zlib/1.2.11

Platform: Windows 10 (build 19042), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Dec 13 2021 21:01:01
	wxWidgets: 3.1.5 (wchar_t,wx containers)
	Boost: 1.76.0
	OCC: 7.5.0
	Curl: 7.78.0-DEV
	ngspice: 35
	Compiler: Visual C++ 1929 without C++ ABI

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON