GameMenu rewiring, FileDialog tweaks

[ajhalme]

• Rewire GameMenu, use signals more consistently
• Fix FileDialog UI event bugs, including Keyboard interacts with game when writing save file name #963
• Fix focus bugs that result in Space key triggering buttons when wanting to end turn / skip unit after clicking menu
• Widen FileDialog a bit, so it's less annoying to use

[stavrosfa]

This looks fine.
I have been working on having more consistent pop ups (with & without advisors) for some time now, but it will be a while before I actually push something.
The scrollbar PR I have opened is kinda part of that, so this is a good oppurtunity to ask, should we keep the Godot native file dialog, or should we re-create the one we see in the original?
I lean towards the latter, since we can have full control over looks & functionality.

[ajhalme]

Building a custom file dialog sounds like a hassle to me.

If anything, on UI matters, I feel like we should more bold in deviating from how things are done in Civ3.