download: reworked connection:

- added shareable code with default proxy, headers and other settings for download tasks like images, symbols, mtgjson, etc;
- use XmageURLConnection.downloadText for text resources
- use XmageURLConnection.downloadBinary for any file resources
- added user agent with app version for all requests;
- added http logs and improved error messages;

Mage.Common/src/main/java/mage/utils/MageVersion.java

@@ -91,4 +91,8 @@ public class MageVersion implements Serializable, Comparable<MageVersion> {
     public boolean isDeveloperBuild() {
         return this.buildTime.contains(JarVersion.JAR_BUILD_TIME_FROM_CLASSES);
     }
+
+    public String getBuildTime() {
+        return this.buildTime;
+    }
 }