Make image file extension filtering case-insensitive

Replace wxImage::GetImageExtWildcard() with a new FILEEXT::ImageFileWildcard()
function that uses KiCad's formatWildcardExt() to generate case-insensitive
file patterns on GTK (e.g., [pP][nN][gG] instead of just png).

This allows selecting image files with uppercase extensions like .JPG or .PNG
in the Place Image dialogs across eeschema, pcbnew, pl_editor, and
bitmap2component.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/21254

bitmap2component/bitmap2cmp_frame.cpp

@@ -399,8 +399,7 @@ void BITMAP2CMP_FRAME::OnLoadFile()
         path = m_mruPath;
 
     wxFileDialog fileDlg( this, _( "Choose Image" ), path, wxEmptyString,
-                          _( "Image Files" ) + wxS( " " )+ wxImage::GetImageExtWildcard(),
-                          wxFD_OPEN | wxFD_FILE_MUST_EXIST );
+                          FILEEXT::ImageFileWildcard(), wxFD_OPEN | wxFD_FILE_MUST_EXIST );
 
     if( fileDlg.ShowModal() != wxID_OK )
         return;