Fix GUI downloads status update

[drew2a]

This PR fixes #7318

The biggest part of this PR is a refactoring of DLSTATUS. This change introduces Status(Enum) which makes it possible to write stricter code and reduce duplication of the status definitions.

Also, a new field was added to json response of async def get_downloads(self, request) to reduce unnecessary conversions from a status code to a status string and back. In simple words this change makes it possible to remove the following function:

    def get_raw_download_status(self):
        return dlstatus_strings.index(self.download_info["status"])

The bugfix itself is the following change (simplified):

from:

    def on_download_stopped(self, json_result):
        if json_result and "modified" in json_result:
            for selected_item in self.selected_items:
                if selected_item.download_info["infohash"] == json_result["infohash"]:
                    selected_item.download_info['status'] = "DLSTATUS_STOPPED"
                    selected_item.update_item()
                    self.update_downloads()

to:

    def on_download_stopped(self, json_result):
        if json_result and "modified" in json_result:
            for selected_item in self.selected_items:
                if selected_item.download_info["infohash"] == json_result["infohash"]:
                    selected_item.update_item()
                    self.update_downloads()

The root cause of the bug is the resynchronization of a download state between the GUI and the core.

[kozlovsky]

Looks good; I only have minor comments about naming

[kozlovsky]

The DLSTATUS_XXX statuses are also described in the doc/restapi/introduction.rst file, probably should be renamed here as well. Also, I see they are mentioned in comments in DownloadState.get_progress & get_status