Wrap NSUserDefaults interactions in properties

Hides the implementation details a little bit better.
Move IDEQuickHelpActionManager overrides to their own file
Rewrite README introduction

Hopefully make the plugin's purpose more clear, and explain its
authorship better.
Tweak menu item wordings
Split one menu item into two

Makes the resulting behavior clearer.
Reword menu items to be a little clearer
Hold Shift to reverse openInDashDisabled flag
Added a tiny change to the title