Add url support (downloading) to /plugins install

Additional changes include code refactoring.

src/tools/http_download.h

@@ -57,6 +57,7 @@ typedef struct http_download_t
     ProfWin* window;
     pthread_t worker;
     int cancel;
+    gboolean silent;
 } HTTPDownload;
 
 void* http_file_get(void* userdata);