Adventure: Fixes and improvements for Save/Load scene and keyboard selection

[EfourC]

This PR fixes various things, mainly in SaveLoadScene, but a few other places as well:

Edit: Also fixes the annoying bug where the save list scrolls to the bottom automatically when entering the Save/Load menu.

• Fix memory leaks relating to the preview images of saves.
• Improve Auto/Quick save slot name text handling. (E.g. after toggling the sort mode, 'Auto Save' button would become 'auto save'.)
• Fix changing selection of save slots with keyboard. Navigating with keyboard was faulty because refreshSaveSlots() didn't clear previous 'selectables' list before deleting and recreating the buttons. This left all previously created buttons still in memory as possible selectables, but they weren't the actual buttons being displayed in the scene any more, causing logic problems (and another memory leak).
• Fix incorrect vertical offset when scrolling selectables with keyboard that are in a ScrollPane (various Scenes). This problem would leave the selected item outside the visible area at the top when scrolling up, or 'too far' away from the bottom when scrolling down.
• Add usable toString()'s' for some UI controls.
• Fix re-selecting previously selected save slot when re-entering the Save/Load scene or toggling sort method.
• Disable the Auto/Quick save slot buttons when in 'Save' mode.

Any critique or other feedback welcome!