Simplify check for orphaned editor.

Instead of rewiring signals on tab.shutting_down, have the webelem check
if its parent tab is deleted, and throw a specific exception.

This is only necessary in WebEngine, Webkit does not crash when the
editor is orphaned.

I tried to write a test for is_deleted, but could not get it to pass:
```
def test_is_deleted(qtbot, view, config_stub, tab_registry, mode_manager):
    tab_w = Tab(win_id=0, mode_manager=mode_manager)
    qtbot.add_widget(tab_w)
    tab_w._set_widget(view)
    assert not tab_w.is_deleted()
    sip.delete(view)
    #assert tab_w.is_deleted()
```

The qtbot post-test cleanup would error due to the deleted view.

qutebrowser/browser/browsertab.py

@@ -22,6 +22,7 @@
 import enum
 import itertools
 
+import sip
 import attr
 from PyQt5.QtCore import pyqtSignal, pyqtSlot, QUrl, QObject, QSizeF, Qt
 from PyQt5.QtGui import QIcon
@@ -864,3 +865,6 @@ class AbstractTab(QWidget):
         except (AttributeError, RuntimeError) as exc:
             url = '<{}>'.format(exc.__class__.__name__)
         return utils.get_repr(self, tab_id=self.tab_id, url=url)
+
+    def is_deleted(self):
+        return sip.isdeleted(self._widget)

qutebrowser/browser/commands.py

@@ -1621,9 +1621,6 @@ class CommandDispatcher:
         ed = editor.ExternalEditor(self._tabbed_browser)
         ed.editing_finished.connect(functools.partial(
             self.on_editing_finished, elem))
-        tab = self._current_widget()
-        tab.shutting_down.connect(functools.partial(
-            self.on_editor_orphaned, ed))
         ed.edit(text, caret_position)
 
     @cmdutils.register(instance='command-dispatcher', scope='window')
@@ -1647,14 +1644,11 @@ class CommandDispatcher:
         """
         try:
             elem.set_value(text)
+        except webelem.OrphanedError as e:
+            message.warning('Edited element vanished')
         except webelem.Error as e:
             raise cmdexc.CommandError(str(e))
 
-    def on_editor_orphaned(self, ed):
-        ed.editing_finished.disconnect()
-        ed.editing_finished.connect(
-            lambda: message.warning('Edited element vanished'))
-
     @cmdutils.register(instance='command-dispatcher', maxsplit=0,
                        scope='window')
     def insert_text(self, text):

qutebrowser/browser/webelem.py

@@ -60,6 +60,13 @@ class Error(Exception):
     pass
 
 
+class OrphanedError(Exception):
+
+    """Raised when a webelement's parent has vanished."""
+
+    pass
+
+
 class AbstractWebElement(collections.abc.MutableMapping):
 
     """A wrapper around QtWebKit/QtWebEngine web element.

qutebrowser/browser/webengine/webengineelem.py

@@ -100,6 +100,8 @@ class WebEngineElement(webelem.AbstractWebElement):
 
     def _js_call(self, name, *args, callback=None):
         """Wrapper to run stuff from webelem.js."""
+        if self._tab.is_deleted():
+            raise webelem.OrphanedError("Tab containing element vanished")
         js_code = javascript.assemble('webelem', name, self._id, *args)
         self._tab.run_js_async(js_code, callback=callback)