Migrate a few deprecated methods to reduce build warnings

[tolmasky]

In this PR I've tried to start tackling some of the deprecated methods that are generating a bunch of warnings in XCode. Specifically I've fixed:

1. I believe all the warnings in FindReplaceController.m (updated beginSheetModalForWindow:)
2. I believe all the warnings in PlainTextWindowController.m (updated beginSheetModalForWindow:)
3. I believe all the warnings in SEEDocumentController.m (beginSheetModalForWindow: and alertWithMessageText:)
4. I've also converted a bunch of ints and longs to NSInteger and NSUInteger as I had that warning turned on before. This is pretty straight forward most the time, except I've changed a (a == b - 1) to (a + 1 == b) here in order to be able to use NSUInteger.

This is a partial fix for #66.

[tolmasky]

I have gone ahead and replaced all the instances of -insertText: with -insertText:replacementRange:. There are now only 10 warnings in the SubEthaEdit project, all related to an alert that isn't as trivial to update.

[tolmasky]

There were a couple more that were easy, down to 6 warnings. I think I'll stop here for this PR and do the other ones in a separate PR.

[monkeydom]

Thanks! Generally speaking I'm not too fond of the NSUInteger for count changes
a) if count exceeds NSInteger the current collection classes fail because of different reasons, so the amount isn't needed
b) lots of subtle unhappy things can happen with unsigned types. So generally I'd rather leave stuff as it is or convert it to signed variants instead of just making the compiler happy bout inbound NSUInteger typed methods, like e.g. count.

That being said: your fixes all look sound.