History Pane rewrite.

The new History Pane features proper mouse and keyboard navigation,
multiple selection and ability to remove the selected entries,
plus a dynamic context menu. Additionaly, the History's size is now
configurable in Preferences.

Use Ctrl+H to show/hide the History Pane.

History Pane's titlebar can be styled via #historyPaneTitleBar, e.g.:

/* Colored header for the History Pane */
  background: lightsteelblue;
  margin: 2px;
}

Closes #162: Make History sidebar independent from the Search Pane
Closes #159: "Send to main window" button from Pop-Up window does nothing when History is shown
Closes #158: Preserve History mode after restart
Closes #157: History with new UI: New words are not added when History is shown
Closes #156: History with new UI: Extra Groups widget in History is shown
Closes #155: History with new UI: Arrow keys navigation in History doesn't work
Closes #154: History with new UI: DEL key does not delete the current history entry