Make DragFileView a subclass of NSBox

- Has same visual appearance
- Does not need to access underlying CALayer
- Prepares for Mojave Dark Mode (Issue #22)

Author: dempseyatgithub

BuildSettingExtractor/DragFileView.h

@@ -9,7 +9,7 @@
 #import <Cocoa/Cocoa.h>
 
 IB_DESIGNABLE
-@interface DragFileView : NSView
+@interface DragFileView : NSBox
 
 @property (readonly) NSURL *fileURL;
 

BuildSettingExtractor/DragFileView.m

@@ -18,9 +18,10 @@ @interface DragFileView ()
 @implementation DragFileView
 
 - (void)commonInit {
-    self.wantsLayer = YES;
+    self.boxType = NSBoxCustom;
+    self.cornerRadius = 20.0;
+    self.borderWidth = 0.0;
     [self setHighlight:NO];
-    self.layer.cornerRadius = 20.0;
     [self registerForDraggedTypes:@[(NSString *)kUTTypeFileURL]];
 }
 
@@ -34,9 +35,9 @@ - (instancetype)initWithFrame:(NSRect)frameRect {
 
 - (void)setHighlight:(BOOL)flag {
     if (flag) {
-        self.layer.backgroundColor = [[NSColor colorWithCalibratedRed:0.56 green:0.7 blue:0.81 alpha:1.0] CGColor];
+        self.fillColor = [NSColor colorWithCalibratedRed:0.56 green:0.7 blue:0.81 alpha:1.0];
     } else {
-        self.layer.backgroundColor = [[NSColor colorWithCalibratedRed:0.7 green:0.85 blue:1.0 alpha:1.0] CGColor];
+        self.fillColor = [NSColor colorWithCalibratedRed:0.7 green:0.85 blue:1.0 alpha:1.0];
     }
 }