Harmonize coding style and whitespace
Reorganized Dialog.mm

Mainly ensure that the class implementations follows their interface declarations.
Remove commented out code
Remove workaround for signatureWithObjCTypes:

This method was made available in 10.5 along with documentation.
NSAutoReleasePool → @autoreleasepool
Update to ARC
Use Foundation (Core) data types
Use Objective-C literals
Remove mention of Carbon
Use new APIs for reading/writing plists

The previous APIs were deprecated because they do not follow proper Cocoa memory management rules.
Do not use the C++ `not` operator
Remove unused `resultCode = 0` assignment

We could make this more terse by returning early, but the surrounding methods all share a similar style.
Remove dead code for sheets

At this point, if we decide to add support for sheets based alerts (nibs) we should add it to the newer Dialog2 plugin.

Along this same line, remove the commented text describing the usage for sheets too.