Minor HIG fixes for Exit/Remove/Delete alerts

[LucaFerretti]

Here is a (maybe dirty) patch to:

• Use <big> markup for primary text in Exit/Remove/Delete? confirmation alert (HIG)
• Always use WARNING stock icon (HIG, QUESTION icon is "deprecated")
• Add a secondary text to Exit alert (HIG, warning alerts should provide it)
• Remove the checkbox from Exit

I'm not totally sure about the last one, but we have yet the option in preferences dialog, and we prompt this only if there are some active torrents, so could be safer don't expose the fact you can skip this alert.

Also, could be good add more info on secondary messages for Remove and Delete alerts, like "if you delete downloaded files, they will be permanently lost"..

[charles]

I like about half of this patch.

• I don't see where the HiG spells out that primary text needs to be wrapped in <big> ... </big>, but since it's that way in the screenshot examples, I'm willing to go with it in r5596.

• The HiG is explicit about needing the warning icon, so that's in r5596 too.

• I don't like the secondary text: it's too wordy, so people won't read it, and it doesn't add anything that wasn't already clear. Also I don't see where confirmation alerts are required to have a secondary text.

• The "don't ask me again" prompt needs to stay in there because it's useful. In fact, reading over the HiG I'm not sure the quit dialog meets the criteria for any of these alerts. It's not a save confirmation, since you won't lose any work when you exit, and it's not a regular confirmation, since it doesn't take 30 seconds of effort to restart Transmission.