implement optional download-open handler setting

- adds new `general` -> `default-open-dispatcher` setting (string)
- if set, will be used instead of QDesktopServices.openUrl (xdg-open backend)
- fixes the 'download handlers' part of issue #841
- note that this is only relevent to the `:download-open` command and other
  methods of opening downloaded files from qutebrowser

doc/help/settings.asciidoc

@@ -11,6 +11,7 @@
 |<<general-ignore-case,ignore-case>>|Whether to find text on a page case-insensitively.
 |<<general-startpage,startpage>>|The default page(s) to open at the start, separated by commas.
 |<<general-yank-ignored-url-parameters,yank-ignored-url-parameters>>|The URL parameters to strip with :yank url, separated by commas.
+|<<general-default-open-dispatcher,default-open-dispatcher>>|The default program used to open downloads. Set to an empty string to use the default internal handler.
 |<<general-default-page,default-page>>|The page to open if :open -t/-b/-w is used without URL. Use `about:blank` for a blank page.
 |<<general-auto-search,auto-search>>|Whether to start a search when something else than a URL is entered.
 |<<general-auto-save-config,auto-save-config>>|Whether to save the config automatically on quit.
@@ -331,6 +332,14 @@ The URL parameters to strip with :yank url, separated by commas.
 
 Default: +pass:[ref,utm_source,utm_medium,utm_campaign,utm_term,utm_content]+
 
+[[general-default-open-dispatcher]]
+=== default-open-dispatcher
+The default program used to open downloads. Set to an empty string to use the default internal handler.
+
+Any {} in the string will be expanded to the filename, else the filename will be appended.
+
+Default: empty
+
 [[general-default-page]]
 === default-page
 The page to open if :open -t/-b/-w is used without URL. Use `about:blank` for a blank page.

qutebrowser/browser/downloads.py

@@ -512,8 +512,8 @@ class AbstractDownloadItem(QObject):
         Args:
             cmdline: The command to use as string. A `{}` is expanded to the
                      filename. None means to use the system's default
-                     application. If no `{}` is found, the filename is appended
-                     to the cmdline.
+                     application or `default-open-dispatcher` if set. If no
+                     `{}` is found, the filename is appended to the cmdline.
         """
         assert self.successful
         filename = self._get_open_filename()
@@ -521,13 +521,22 @@ class AbstractDownloadItem(QObject):
             log.downloads.error("No filename to open the download!")
             return
 
-        if cmdline is None:
+        # the default program to open downloads with - will be empty string
+        # if we want to use the default
+        override = config.get('general', 'default-open-dispatcher')
+
+        # precedence order: supplied cmdline > default-open-dispatcher > openUrl
+
+        if cmdline is None and not override:
             log.downloads.debug("Opening {} with the system application"
                                 .format(filename))
             url = QUrl.fromLocalFile(filename)
             QDesktopServices.openUrl(url)
             return
 
+        if cmdline is None and override:
+            cmdline = override
+
         cmd, *args = shlex.split(cmdline)
         args = [arg.replace('{}', filename) for arg in args]
         if '{}' not in cmdline:

qutebrowser/config/configdata.py

@@ -147,6 +147,13 @@ def data(readonly=False):
             "The URL parameters to strip with :yank url, separated by "
             "commas."),
 
+            ('default-open-dispatcher',
+             SettingValue(typ.String(none_ok=True), ''),
+            "The default program used to open downloads. Set to an empty "
+            "string to use the default internal handler.\n\n"
+            "Any {} in the string will be expanded to the filename, else "
+            "the filename will be appended."),
+
             ('default-page',
              SettingValue(typ.FuzzyUrl(), '${startpage}'),
              "The page to open if :open -t/-b/-w is used without URL. Use "