Better UX of the click mode setting

[artginzburg]

This basically matches MiddleClick's interface with System Settings > Trackpad > Point & Click.

• The click mode reads its state from System Settings, unless the user manually changes it.
• Instead of 2 menu items — each for each click mode, there is now one — "Tap to click", which is checked/unchecked accordingly.
• There's also an info menu item that shows current user settings.
• The click mode can be reset to match System Settings by clicking the "Tap to click" menu item while holding the Option key.