eeschema: Add "Preview changes" button for "Tools" -> "Annotate Schematic"

Description

It would be nice to get the list of pending changes to designators before actually clicking "Annotate" button.

Steps to reproduce

1. Tools -> "Annotate schematic..";
2. Wonder what is going to happen after button "Annotate" click;

It would be nice to have idea of what is going to happen by adding "Preview changes" button which would provide the same log output as would be seen after "Annotate" button click, but without actual changes made to the schematic.

KiCad Version

Application: Eeschema

Version: (5.99.0-8964-g20338c92e8), release build

Libraries:
	wxWidgets 3.1.4
	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: Feb 10 2021 09:34:08
	wxWidgets: 3.1.4 (wchar_t,STL containers)
	Boost: 1.75.0
	OCC: 7.4.0
	Curl: 7.74.0-DEV
	ngspice: 33
	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