Paint's Dark GUI

Changes since v3.2:

* Changes to HUD:
     * Hotbar & off-hand slot textures added.
     * Vanilla Animal Hearts are now Horseshoe Hearts.
     * Empty experience bar now greener.
 * Changes to GUIs:
     * Hoppers and Hopper Minecarts' GUI name background now extends further because "Minecart with Hopper" was too long for the background.
     * Changed gamemode switcher to be more in the style of Paint's Dark GUI.

* Changes to Creative Mode Inventory:
     * Changed Inventory tabs to make more sense with this style.
     * Search field is now aqua to match the recipe book search field.

* Changes made because of 1.17:
     * Frozen Hearts and Hardcore Frozen Hearts added.
     * pack_format is now 7.

This pack was made in 21w14a, but if there are no more changes that need to be made before 1.17, then this will be the 1.17 version.