Don't ask again on close

Hey 
i added checkbox which can user use to determine if he want confrimation dialog when closing application.
Checbox can be checked in Settings page or directly on dialog. 
Here is how it looks now:
![closedialog](https://f.cloud.github.com/assets/3951334/2435328/91413334-adc4-11e3-9368-8e5c68666b94.png)
I also centered buttons because with checkbox it looked ugly.
Maybe checkbox should be in the same line with buttons. What do you think?
Change in settings page:
![settingswithclosedialog](https://f.cloud.github.com/assets/3951334/2435329/99e40f5c-adc4-11e3-866d-13ac719d17d3.png)

I also fixed tray label. When user minimized window by clicking on close button, tray had label "hide" instead of "show"