Still needs more work to make it safer (especially entering backslashes will give YSODs and sometimes when you move to a different folder the old file stays behind)
will cause exception when Masterpages folder contain a directory without skin.xml
possibly caused by unclean skin uninstall
src/Umbraco.Web/umbraco.presentation/umbracoLiveEditing\Modules\SkinModule\SkinCustomizer.ascx.cs