Support PNG, JPEG, BMP, WEBP export formats by change of "Export to PNG" menu item into "Export" with its own pull-down dialog (and a HTML5 range type input element for "quality" selection for JPEG or WEBP files), including changes for the server_opensave extension; change locale key from "export_png" to "export_img" and change the corresponding localized strings (currently CS, DE, ES, FR, IT, NL, PT-BR, RO, SK, all reset to English to avoid translation errors); within the setCustomHandlers() API, deprecate "pngsave" in favor of "exportImage" (chosen to avoid just using the simpler but reserved JS keyword "export") including use within the server_opensave extension; a few JSLint-friendly changes

git-svn-id: http://svg-edit.googlecode.com/svn/trunk@2602 eee81c28-f429-11dd-99c0-75d572ba1ddd