s:ProcessSign does not handle new sign output

[blueyed]

Code ref: https://github.com/tomtom/quickfixsigns_vim/blob/653cf16034cdbbe640e63ac9698f7da587c0fa56/plugin/quickfixsigns.vim#L875

Example sign-place output:

['--- Signs ---', 'Signs for foo:', '    line=99  id=3000  name=GitGutterLineModified  priority=10', '    line=100  id=3001  name=GitGutterLineAdded  priority=10' …

It should probably use the new sign_getplaced() there.