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/ghdl.cpp

@@ -233,28 +233,24 @@ Menu::Menu() : MenuBase{"GitHub"_i18n} {
     fs::FsNativeSd().CreateDirectoryRecursively(CACHE_PATH);
 
     this->SetActions(
-        std::make_pair(Button::R2, Action{[this](){
-        }}),
-
         std::make_pair(Button::DOWN, Action{[this](){
-            if (m_index < (m_entries.size() - 1)) {
-                SetIndex(m_index + 1);
-                App::PlaySoundEffect(SoundEffect_Scroll);
-                if (m_index - m_index_offset >= 8) {
-                    log_write("moved down\n");
-                    m_index_offset++;
-                }
+            if (ScrollHelperDown(m_index, m_index_offset, 1, 1, 8, m_entries.size())) {
+                SetIndex(m_index);
             }
         }}),
-
         std::make_pair(Button::UP, Action{[this](){
-            if (m_index != 0) {
-                SetIndex(m_index - 1);
-                App::PlaySoundEffect(SoundEffect_Scroll);
-                if (m_index < m_index_offset ) {
-                    log_write("moved up\n");
-                    m_index_offset--;
-                }
+            if (ScrollHelperUp(m_index, m_index_offset, 1, 1, 8, m_entries.size())) {
+                SetIndex(m_index);
+            }
+        }}),
+        std::make_pair(Button::DPAD_RIGHT, Action{[this](){
+            if (ScrollHelperDown(m_index, m_index_offset, 8, 1, 8, m_entries.size())) {
+                SetIndex(m_index);
+            }
+        }}),
+        std::make_pair(Button::DPAD_LEFT, Action{[this](){
+            if (ScrollHelperUp(m_index, m_index_offset, 8, 1, 8, m_entries.size())) {
+                SetIndex(m_index);
             }
         }}),