added ability to quit a session

Ignore-this: 8136283786c3164cfa8e2d1ed9953a41

darcs-hash:20130315025042-c21fe-5e702a85e09e4aa28fa0c1d77e8f4feb785a88ee
sessions specific menu items are enabled and disabled as you click tabs

Ignore-this: 4062d4f3a57b1e47e7fd4ec942d25211

darcs-hash:20130315030153-c21fe-259a8d69b02b218fb19f730f37438db41d44ad4f
added dialogs for quiting and removing

Ignore-this: edb0d8a0adbd073d749a32e77e2387de

added dialogs for making sure the user wants to quit a session, remove a contact, or
remove a greeter

darcs-hash:20130315031718-c21fe-44814e741e6e6bb61745d32db291f017c7a5931b