Factor out status bar into a separate class

[eric@adblockplus.org]

Background

CPluginClass is something of a ​god object. All of the code for the status bar icon and its associated menu is part of this class at present.

What to change

• Extract all the status bar code into a separate class.
• If possible, share a single status bar object between multiple instances of CPluginClass.
• Ensure the life cycle of the new class is in harmony with CPluginClass
• Define clear responsibilities for the new class and the remnant CPluginClass
  • The status bar class is responsible for correctly displaying its status relative to the current tab. It does not have responsibility for determining that status; it asks another class.

[eric@adblockplus.org]

This relates to issue #3382 to rationalize the life cycle of some of the user interface members of CPluginClass.

[eric@adblockplus.org]

This relates to #712, which is a problem with managing the display state of the icon. It would be possible to complete #712 without addressing the present issue, but only by making CPluginClass more of a god object. Thus this issue has been marked as blocking #712.