value-field of power symbols can be written

Description

The value-field of power-symbols can be written in the "Edit Value Field"-dialog. According to a note (and to the v6-behaviour) this should not be possible (as the net-names in v7 are derived from the pin-name and difference between invisible pin-name and displayed value creates only problems).

This affects not the power symbol properties dialog - there the value-field is correctly write-protected.

Steps to reproduce

1. add a power-symbol to a schematic
2. doubleclick the value --> "Edit Value Field" dialog opens
3. observe the note: value field should be write-protected
4. instead it's a normal writeable field (see picture 2)

KiCad Version

Application: KiCad Schematic Editor x64 on x64

Version: (7.0.0-18-ga48eace90a), release build

Libraries:
	wxWidgets 3.2.1
	FreeType 2.12.1
	HarfBuzz 5.0.1
	FontConfig 2.14.1

Platform: Windows 10 (build 19044), 64-bit edition, 64 bit, Little endian, wxMSW

	wxWidgets: 3.2.1 (wchar_t,wx containers)
	Boost: 1.80.0
	OCC: 7.6.2
	Curl: 7.83.1-DEV
	ngspice: 39
	Compiler: Visual C++ 1934 without C++ ABI

Build settings:
	KICAD_SPICE=ON