[Godot 4] Fix Popup Bugs

[pcen]

Incrementally removing bugs from the godot4 branch:

• prevents polling for input actions in Game.cs when a popup is visible
• fixes behaviour when the build city popup is open and esc. is pressed: before it would close the popup and show the exit game popup at the same time. now, the first esc. press will only close the current popup

[pcen]

discovered #424 while cleaning up this change

[Sean-Brown]

Getting back into this and not very familiar with all the SDKs involved, but the code changes still make sense to me.

[QuintillusCFC]

Reviewed code, the changes make sense, and should prevent the multi-popup issue generally. Approved and merging.