Move MMainWnd declaration to MMainWnd.hpp and preview implementations to separate compilation unit (#340)

* Initial plan

* Move preview-related implementations to MMainWnd_Preview.cpp

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

* Fix: Include MMainWnd_Preview.cpp from RisohEditor.cpp since MMainWnd class is defined there

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

* Move MMainWnd declaration from RisohEditor.cpp to MMainWnd.hpp

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

* Remove unnecessary MMainWnd_Preview.h file

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

* Fix CI: Add IMPORT_RESULT enum to header and add missing includes

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

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
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