Motivation / Problem

Since f6d5c01, windows' deletion is immediate.
ProcessScheduledInvalidations() can do invalid reads if window is self deleted during OnInvalidateData().
And probably other invalid accesses are yet to be found.

Description

Replace delete this used in Window event handlers, with this->Close() which hides the window, but delay the real deletion so it happens in a safe place.

Limitations

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 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