pcbnew: bitmap image lock state is not saved

Description

Bitmap image lock state is not saved in pcbnew files.

Steps to reproduce

1. Open pcbnew app and add a bitmap image(I have tried a PNG file with alpha channel).
2. Lock the image and save the file.
3. Close the file and open it again.
4. You should see the image is not locked anymore.

KiCad Version

Application: KiCad PCB Editor x64 on x64

Version: 7.0.9, release build

Libraries:
	wxWidgets 3.2.3
	FreeType 2.12.1
	HarfBuzz 8.2.1
	FontConfig 2.14.2
	libcurl/8.4.0-DEV Schannel zlib/1.3

Platform: Windows 11 (build 22621), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Nov  5 2023 19:26:40
	wxWidgets: 3.2.3 (wchar_t,wx containers)
	Boost: 1.83.0
	OCC: 7.7.1
	Curl: 8.4.0-DEV
	ngspice: 41
	Compiler: Visual C++ 1936 without C++ ABI

Build settings:
	KICAD_SPICE=ON