Add support for working with local files using a custom file engine. Map the local files to prefixed paths like weblocalfile:/n/filename.ext where "filename.ext" is the file name provided by the native API and "n" is a counter to ensure file path uniqueness (required since the web platform does not provide the file path) There are two parts to this support: 1) QWasmFileDialogHelper: a QPlatformFileDialogHelper subclass which uses the existing QtGui support for native web file dialogs. On file dialog accept each opened file is registered with the file engine. 2) QWasmFileEngine: a QAbstractFileEngine subclass which tracks opened wasm files and maps "webfile" urls to native file handles (either a File or a FileSystemFileHandle, depending on browser support). File read/write is implemented using existing QIODevice subclasses which support read/write using the native async API with asyncify/JSPI. TODO (known issues): - File handle lifetime management. Currently all file handles are leaked. Based on https://codereview.qt-project.org/c/qt/qtbase/+/562724 Pick-to: 6.11 Done-With: Even <even.oscar.andersen@qt.io> Change-Id: Icf143235fdf2071c34c5be3387d6f7b2b096ff80 Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>