Fix #192: Clarify customization is in help dialog

- Add button in settings page to customize keyboard shortcuts (open the
  help dialog)
- Change "Help" to "Keyboard shortcuts"
- Fix bug: Help dialog broke the layout in XUL pages
Merge pull request #195 from lydell/clarify-help-customization

Fix #192: Clarify help customization
Fix #190: Improve blacklisting UI

- Fix bug: on `about:` pages `**` was suggested as blacklisting rule
- On blacklisted pages, the blacklist button un-blacklists instead
- Live validation of the black list setting