Add url support (downloading) to /plugins install

Additional changes include code refactoring.

tests/unittests/tools/stub_plugin_download.c

@@ -0,0 +1,19 @@
+#ifndef TOOLS_PLUGIN_DOWNLOAD_H
+#define TOOLS_PLUGIN_DOWNLOAD_H
+
+#include <stdlib.h>
+typedef struct prof_win_t ProfWin;
+typedef struct http_download_t HTTPDownload;
+
+void*
+plugin_download_install(void* userdata)
+{
+    return NULL;
+}
+
+void
+plugin_download_add_download(HTTPDownload* download)
+{
+}
+
+#endif