Clean up and refactor IngameWindow button handling

[falbrechtskirchinger]

• Remove unused data members (last_down, last_down2).
• Refactor button handling for Allow excluding windows from being closed via Escape key #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.

[Flamefire]

Looks good, only change is basically to use an enum class throughout.

I added pointers how to make best use of that, if anything is unclear, feel free to ask.

[falbrechtskirchinger]

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

[Flamefire]

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

Shame but unfortunately true :(

Last bit of polishing is mostly getting rid of the remnants of the button ID: Rename the enum and all variables of that type (from ìdto e.g.buttonorbt`)

[Flamefire]

Thanks!