wasm: add Window.localStorage settings backend

Window.localStorage provides a synchronous API for storing
saving data across browsing sessions, and is a good match
for QSettings. Storage is limited to 5MB per origin, which
should be sufficient for typical application settings.

Window.localStorage is shared by all pages/apps on the
origin (e.g. "https://qt.io" is one origin), which makes
key collisions possible. To avoid this the key structure
is "qt-v0-org-app-userkey", where both the organization
and application name is used to differentiate between
keys, and "v0" is a version tag to allow changing the
key structure in the future. We reserve the "qt" prefix
for keys written by QSettings.

Add the new implementation as QWasmLocalStorageSettingsPrivate,
rename the existing settings backend to QWasmIDBSettingsPrivate.

Make QSettingsPrivate::create() support backend selection using
the QSettings::Format enum. It now also supports selecting the
Ini backend, which can be used to store larger amounts of settings,
for example to a file on IDBFS. (alternatively IDBSettings +
asyncify could also be used for this case)

Change-Id: If70aa488635018218bc2a19803c4a719732c0004
Reviewed-by: David Skoland <david.skoland@qt.io>

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