Make strings translatable

- Use Properties.Resources for the title and message in the new
  MessageBox "Album art too large".
- Show "Downloading album art..." in the status of CUERipper to inform
  the user, what is going on, because downloading of large album art
  may take some time.
- Add German translations.

CUERipper/Properties/Resources.de-DE.resx

@@ -147,4 +147,13 @@
   <data name="Retry" xml:space="preserve">
     <value>erneut versuchen</value>
   </data>
+  <data name="DownloadingAlbumArt" xml:space="preserve">
+    <value>Coverbild wird heruntergeladen …</value>
+  </data>
+  <data name="AlbumArtTooLargeMessage" xml:space="preserve">
+    <value>Das gewählte Coverbild hat eine Größe von {0} Bytes und ist somit zu groß (max. 16 MB). Die kleine Coverbildgröße (Small) wird stattdessen verwendet.</value>
+  </data>
+  <data name="AlbumArtTooLargeTitle" xml:space="preserve">
+    <value>Coverbild zu groß</value>
+  </data>
 </root>

CUERipper/Properties/Resources.resx

@@ -223,4 +223,13 @@
   <data name="opus" type="System.Resources.ResXFileRef, System.Windows.Forms">
     <value>..\Resources\opus.ico;System.Drawing.Icon, System.Drawing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a</value>
   </data>
+  <data name="DownloadingAlbumArt" xml:space="preserve">
+    <value>Downloading album art...</value>
+  </data>
+  <data name="AlbumArtTooLargeMessage" xml:space="preserve">
+    <value>Selected album art has a size of {0} bytes, which is too large for embedding (max 16 MB). Small album art will be used instead.</value>
+  </data>
+  <data name="AlbumArtTooLargeTitle" xml:space="preserve">
+    <value>Album art too large</value>
+  </data>
 </root>

CUERipper/frmCUERipper.cs

@@ -627,15 +627,14 @@ private void buttonGo_Click(object sender, EventArgs e)
                     var ms = new MemoryStream();
                     try
                     {
+                        toolStripStatusLabel1.Text = Properties.Resources.DownloadingAlbumArt;
                         if (cueSheet.CTDB.FetchFile(albumArt[currentAlbumArt].meta.uri, ms))
                         {
                             if (ms.Length < 0xffffff || !_config.embedAlbumArt)
                                 albumArt[currentAlbumArt].contents = ms.ToArray();
                             else
-                                MessageBox.Show(this, String.Format("Selected album art has a size of {0} bytes, " +
-                                "which is too large for embedding (max 16 MB). " +
-                                "Small album art will be used instead.", ms.Length),
-                                "Album art too large", MessageBoxButtons.OK, MessageBoxIcon.Warning);
+                                MessageBox.Show(this, String.Format(Properties.Resources.AlbumArtTooLargeMessage, ms.Length),
+                                Properties.Resources.AlbumArtTooLargeTitle, MessageBoxButtons.OK, MessageBoxIcon.Warning);
                         }
                     } catch (Exception)
                     {