Fix #11115: Focus the abandon game/exit game windows

[merni-ns]

Motivation / Problem

Fixes #11115.

Description

• Add a parameter to ShowQuery which gives focus to the query window created by the function. By default it is false to preserve current behaviour.
• Use that parameter to focus the "Abandon game" and "Exit game" confirmation queries.
• Also, the query created by ShowQuery is not modal even though it is described as modal by the comment. So that comment is fixed.

Limitations

Opening the abandon/exit query windows will now remove focus on whatever window is currently focussed.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[2TallTyler]

We could probably bikeshed about what other windows could or should be focused...but let's not. The new default parameter is a nice way to handle this, I think.