Added paste-primary command

The Shift+Ins key should arguably insert primary selection, not the
clipboard selection as every Qt program does. This commit makes it
possible via the hidden paste-primary command (enabled by default).

Unfortunately QtWebKit does not provide any straightforward way to
insert text at cursor position into editable fields, so we work around
this by executing a JavaScript snippet - inspired by this SO answer:
http://stackoverflow.com/a/11077016

qutebrowser/browser/commands.py

@@ -1307,6 +1307,39 @@ class CommandDispatcher:
         except webelem.IsNullError:
             raise cmdexc.CommandError("Element vanished while editing!")
 
+    @cmdutils.register(instance='command-dispatcher',
+                       modes=[KeyMode.insert], hide=True, scope='window')
+    def paste_primary(self):
+        """Paste the primary selection at cursor position into the curently
+        selected form field.
+        """
+        frame = self._current_widget().page().currentFrame()
+        try:
+            elem = webelem.focus_elem(frame)
+        except webelem.IsNullError:
+            raise cmdexc.CommandError("No element focused!")
+        if not elem.is_editable(strict=True):
+            raise cmdexc.CommandError("Focused element is not editable!")
+
+        clipboard = QApplication.clipboard()
+        if clipboard.supportsSelection():
+            sel = clipboard.text(QClipboard.Selection)
+            log.misc.debug("Pasting selection: '{}'".format(sel))
+            elem.evaluateJavaScript("""
+                var sel = '%s';
+                if (this.selectionStart || this.selectionStart == '0') {
+                    var startPos = this.selectionStart;
+                    var endPos = this.selectionEnd;
+                    this.value = this.value.substring(0, startPos)
+                        + sel
+                        + this.value.substring(endPos, this.value.length);
+                    this.selectionStart = startPos + sel.length;
+                    this.selectionEnd = startPos + sel.length;
+                } else {
+                    this.value += sel;
+                }
+                """ % sel)
+
     def _clear_search(self, view, text):
         """Clear search string/highlights for the given view.
 

qutebrowser/config/configdata.py

@@ -1324,7 +1324,8 @@ KEY_SECTION_DESC = {
         "Since normal keypresses are passed through, only special keys are "
         "supported in this mode.\n"
         "Useful hidden commands to map in this section:\n\n"
-        " * `open-editor`: Open a texteditor with the focused field."),
+        " * `open-editor`: Open a texteditor with the focused field.\n"
+        " * `paste-primary`: Paste primary selection at cursor position."),
     'hint': (
         "Keybindings for hint mode.\n"
         "Since normal keypresses are passed through, only special keys are "
@@ -1495,6 +1496,7 @@ KEY_DATA = collections.OrderedDict([
 
     ('insert', collections.OrderedDict([
         ('open-editor', ['<Ctrl-E>']),
+        ('paste-primary', ['<Shift-Ins>']),
     ])),
 
     ('hint', collections.OrderedDict([