Some improves on "Edit Text and Graphic Properties"

Description

I just missed tow characteristics that can be improved on the "Edit Text and Graphic Properties" at Pcbnew.

Filter layers by more than one layer. This will need to change the dialog or at lead the combobox object to a list-combobox...
When delete a numeric value (using <delete>) it always get 0mm, I expected the "--leave unchanged --" value. This could be classified as a minor issue.

Steps to reproduce

Open a board with different size text objects;
Some properties are "--leave unchanged --" on "Edit Text and Graphic Properties" due its different values;
Type some new thickness/width/height... by mistake;
You want now to delete this value and leave it as "unchanged" but you can't.

KiCad Version

Application: Pcbnew

Version: 5.99.0-unknown-50e22de3ba~95~ubuntu20.04.1, release build

Libraries:
	wxWidgets 3.0.4

Platform: Linux 5.8.0-38-generic x86_64, 64 bit, Little endian, wxGTK, ubuntu, x11

	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.71.0
	OCC: 7.3.0
	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
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON