Clean up and refactor IngameWindow button handling

[falbrechtskirchinger]

• Remove unused data members (last_down, last_down2).
• Refactor button handling for #1606.
• Fix incorrect texture used for the close button (pressed state).

I'll rebase #1606 and implement discussed changes before this is ready for consideration.

[falbrechtskirchinger]

Had to move the enum out of the class, as maxEnumValue(ButtonId) isn't usable in an incomplete class context.