Move preview-related implementations to MMainWnd_Preview.cpp

Co-authored-by: katahiromz <2107452+katahiromz@users.noreply.github.com>

Author: Copilot

src/CMakeLists.txt

@@ -21,6 +21,7 @@ set_target_properties(libRisohEditor PROPERTIES PREFIX "")
 # RisohEditor.exe
 add_executable(RisohEditor WIN32
     RisohEditor.cpp
+    MMainWnd_Preview.cpp
     RisohEditor_res.rc
     res/Manifest_1.manifest)
 target_link_libraries(RisohEditor
@@ -35,6 +36,7 @@ endif()
 # RisohEditorPortable.exe
 add_executable(RisohEditorPortable WIN32
     RisohEditor.cpp
+    MMainWnd_Preview.cpp
     RisohEditor_res.rc
     res/Manifest_1.manifest)
 target_link_libraries(RisohEditorPortable