history: Add clear() method and history-clear command

WebHistory now has a clear() method which is also a command
(history-clear) which clears the qutebrowser history using the new
lineparser clear() method and emits a cleared signal.

The completion model urlmodel connects to the WebHistory.cleared signal
and clears its history category completion list.

I am adding this as a temporary fix before #58 or #1051 get implemented.

qutebrowser/browser/history.py

@@ -25,6 +25,7 @@ import collections
 from PyQt5.QtCore import pyqtSignal, QUrl
 from PyQt5.QtWebKit import QWebHistoryInterface
 
+from qutebrowser.commands import cmdutils
 from qutebrowser.utils import utils, objreg, standarddir, log
 from qutebrowser.config import config
 from qutebrowser.misc import lineparser
@@ -72,10 +73,12 @@ class WebHistory(QWebHistoryInterface):
                              arg: The new HistoryEntry.
         item_added: Emitted after a new HistoryEntry is added.
                     arg: The new HistoryEntry.
+        cleared: Emitted after the history is cleared.
     """
 
     add_completion_item = pyqtSignal(HistoryEntry)
     item_added = pyqtSignal(HistoryEntry)
+    cleared = pyqtSignal()
     async_read_done = pyqtSignal()
 
     def __init__(self, parent=None):
@@ -169,6 +172,16 @@ class WebHistory(QWebHistoryInterface):
         self._lineparser.save()
         self._saved_count = len(self._new_history)
 
+    @cmdutils.register(name='history-clear', instance='web-history')
+    def clear(self):
+        """Clear all history entries."""
+        self._lineparser.clear()
+        self._history_dict.clear()
+        self._temp_history.clear()
+        self._new_history.clear()
+        self._saved_count = 0
+        self.cleared.emit()
+
     def addHistoryEntry(self, url_string):
         """Called by WebKit when an URL should be added to the history.
 

qutebrowser/completion/models/urlmodel.py

@@ -76,6 +76,7 @@ class UrlCompletionModel(base.BaseCompletionModel):
             self._add_history_entry(entry)
         self._history.add_completion_item.connect(
             self.on_history_item_added)
+        self._history.cleared.connect(self.on_history_cleared)
 
         objreg.get('config').changed.connect(self.reformat_timestamps)
 
@@ -130,6 +131,10 @@ class UrlCompletionModel(base.BaseCompletionModel):
         else:
             self._add_history_entry(entry)
 
+    @pyqtSlot()
+    def on_history_cleared(self):
+        self._history_cat.removeRows(0, self._history_cat.rowCount())
+
     def _remove_item(self, data, category, column):
         """Helper function for on_quickmark_removed and on_bookmark_removed.