Improve display of blank albums

* Add dark and light blank album image variants
* Use CALayer to draw rounded corners and a tinted border on each album
  cover

Persephone/Assets.xcassets/blankAlbum.imageset/Contents.json

@@ -1,12 +1,35 @@
 {
   "images" : [
+    {
+      "idiom" : "universal"
+    },
     {
       "idiom" : "universal",
-      "filename" : "blankAlbum.pdf"
+      "filename" : "blankAlbumLight-1.pdf",
+      "appearances" : [
+        {
+          "appearance" : "luminosity",
+          "value" : "light"
+        }
+      ]
+    },
+    {
+      "idiom" : "universal",
+      "filename" : "blankAlbumDark.pdf",
+      "appearances" : [
+        {
+          "appearance" : "luminosity",
+          "value" : "dark"
+        }
+      ]
     }
   ],
   "info" : {
     "version" : 1,
     "author" : "xcode"
+  },
+  "properties" : {
+    "template-rendering-intent" : "original",
+    "preserves-vector-representation" : true
   }
 }

Persephone/Resources/AlbumItem.xib

@@ -9,7 +9,9 @@
         <customObject id="-2" userLabel="File's Owner" customClass="AlbumItem" customModule="Persephone" customModuleProvider="target">
             <connections>
                 <outlet property="albumArtist" destination="5Uu-j1-qyT" id="2Et-tX-InT"/>
+                <outlet property="albumCoverView" destination="Kfb-8f-ean" id="ZAL-jD-lHj"/>
                 <outlet property="albumTitle" destination="KEh-NL-c2W" id="SI3-hm-H2B"/>
+                <outlet property="imageView" destination="Kfb-8f-ean" id="Ur0-hX-wJm"/>
                 <outlet property="view" destination="Hz6-mo-xeY" id="v7W-XA-Emc"/>
             </connections>
         </customObject>

Persephone/Views/AlbumItem.swift

@@ -9,9 +9,16 @@
 import Cocoa
 
 class AlbumItem: NSCollectionViewItem {
+  let albumCoverBorderColor = NSColor.init(calibratedWhite: 1, alpha: 0.1)
+
   override func viewDidLoad() {
     super.viewDidLoad()
     // Do view setup here.
+
+    albumCoverView.wantsLayer = true
+    albumCoverView.layer?.cornerRadius = 3
+    albumCoverView.layer?.borderWidth = 1
+    albumCoverView.layer?.borderColor = albumCoverBorderColor.cgColor
   }
 
   func setAlbum(_ album: MPDClient.Album) {
@@ -19,6 +26,7 @@ class AlbumItem: NSCollectionViewItem {
     albumArtist.stringValue = album.artist
   }
 
+  @IBOutlet var albumCoverView: NSImageView!
   @IBOutlet var albumTitle: NSTextField!
   @IBOutlet var albumArtist: NSTextField!
 }