Add :window-only command

I mainly added this so I can speed up all of the tests that rely on
closing other windows.

CHANGELOG.asciidoc

@@ -32,6 +32,7 @@ Added
   in rapid mode.
 - New `{clipboard}` and `{primary}` replacements for the commandline which
   replace the `:paste` command.
+- New `:window-only` command to close all other windows.
 
 Changed
 ~~~~~~~

qutebrowser/browser/commands.py

@@ -769,6 +769,13 @@ class CommandDispatcher:
             raise cmdexc.CommandError("Can't zoom {}%!".format(level))
         message.info(self._win_id, "Zoom level: {}%".format(level))
 
+    @cmdutils.register(instance='command-dispatcher', scope='window')
+    def window_only(self):
+        """Close all windows except for the current one."""
+        for win_id, window in objreg.window_registry.items():
+            if win_id != self._win_id:
+                window.close()
+
     @cmdutils.register(instance='command-dispatcher', scope='window')
     def tab_only(self, left=False, right=False):
         """Close all tabs except for the current one.

tests/end2end/features/misc.feature

@@ -525,6 +525,20 @@ Feature: Various utility commands.
             - data/hints/link_blank.html
             - data/hello.txt (active)
 
+    @no_xvfb
+    Scenario: :window-only
+        Given I run :tab-only
+        And I open data/hello.txt
+        When I open data/hello2.txt in a new tab
+        And I open data/hello3.txt in a new window
+        And I run :window-only
+        Then the session should look like:
+            windows:
+            - tabs:
+              - active: true
+                history:
+                - url: http://localhost:*/data/hello3.txt
+
     ## Variables
 
     Scenario: {url} as part of an argument