AppDelegate: Add NSTextFinder find bar

Add a find bar leveraging NSTextFinder. It turns out I didn't need to do
anything custom with `NSTextFinder` since `NSTextView` conveniently
already has a `usesFindBar` property to turn on built-in support for it.
I discovered that by looking at the TextEdit source code available here:

https://developer.apple.com/library/archive/samplecode/TextEdit/Introduction/Intro.html

I added an `NSScrollView` because you kind of need one to implement
`NSTextFinder`. Not sure if the finding functionality would still work
without the scroll view.

Also use the proper `-performTextFinderAction:` selector for the "Find…"
menu item to open the find bar.

Now we can confirm that the "Find" menu items work correctly.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/AppDelegate.h b/src/AppDelegate.h
index a9570b8..4dba427 100644
--- a/src/AppDelegate.h
+++ b/src/AppDelegate.h
@@ -2,6 +2,8 @@
 
 @interface AppDelegate : NSObject <NSApplicationDelegate> {
 	NSWindow *_window;
+	NSScrollView *_scroll_view;
 	NSTextView *_text_view;
+	// NSTextFinder *_text_finder;
 }
 @end