Add support for transparency and blur from Alfred's theme

`AlfredExtraPane.app` has been updated to support Alfred themes
containing transparency and blur.
 - mr-pennyworth/alfred-extra-pane@16e71e0
 - mr-pennyworth/alfred-extra-pane@2295b98

Given that the new version of the extra pane renders the window
with appropriate color, and expects the HTML to not have its own
background color, in this change, we update the CSS for dict entries.

dict-entry.css

@@ -7,7 +7,7 @@
 
 html
 {
-    background-color: var(--window-color, #1d1e28);
+    background-color: transparent;
     color: var(--result-text-color, #eee);
 }
 

info.plist

@@ -890,7 +890,7 @@ Subsequent searches should be snappy
 	<key>variablesdontexport</key>
 	<array/>
 	<key>version</key>
-	<string>0.2.14</string>
+	<string>0.2.15</string>
 	<key>webaddress</key>
 	<string>https://github.com/mr-pennyworth/alfred-better-dictionaries</string>
 </dict>