fix etag cache not being returned upon the 2nd request, add jump page support for all menus.

- all menus feature page jumping, using L2/R2 (or DPAD_LEFT/DPAD_RIGHT in list menus)
- successive calls to fetch the etag would fail, this was seen in themezer and github menus.
- add limit the number of icons loaded per frame in homebrew menu.
- display default icon the image is not ready to be loaded / invalid.

fixes #53

sphaira/source/ui/menus/main_menu.cpp

@@ -158,12 +158,6 @@ MainMenu::MainMenu() {
                 return false;
             }
 
-            if (result.code == 304) {
-                log_write("data hasn't changed\n");
-            } else {
-                log_write("etag changed\n");
-            }
-
             auto json = yyjson_read_file(CACHE_PATH, YYJSON_READ_NOFLAG, nullptr, nullptr);
             R_UNLESS(json, false);
             ON_SCOPE_EXIT(yyjson_doc_free(json));