symbol editor pin table: allow bulk edit pin length (improvement)

Description

Allow editing several cells at a time:

1003G

Steps to reproduce

  1. Open symbol editor -> open pin table;
  2. Select all pin pin length cells;
  3. Type in new pin length (numeric value);
  4. BUG: Only one cell is changed;
  5. Repeat step 2, hit DEL key;
  6. OK -> all cells are nulled;
  7. Repeat step 2, Paste some numeric value (RMB menu entry);
  8. OK -> all cells are changed.

Expected, correct behavior can be seen in steps 6 and 8. Step 4 should act in the same way as these steps.

KiCad Version

Application: KiCad Symbol Editor (64-bit)

Version: (5.99.0-10721-g0b4075e3a9), release build

Libraries:
	wxWidgets 3.1.5
	libcurl/7.74.0-DEV Schannel zlib/1.2.11

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

Build Info:
	Date: May 31 2021 21:00:57
	wxWidgets: 3.1.5 (wchar_t,STL containers)
	Boost: 1.75.0
	OCC: 7.5.0
	Curl: 7.74.0-DEV
	ngspice: 34
	Compiler: Visual C++ 1928 without C++ ABI

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
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON
Edited by Jeff Young