Add Apply button to preferences

[damyan94]

Summary

• Add an Apply button to the Preferences window.
• Apply configuration changes without closing Preferences.
• Keep the user on the currently selected page and profile.
• Reuse the same Apply path when closing Preferences.

This makes it easier to adjust settings interactively without repeatedly
closing and reopening the Preferences window.

Verification

• Python syntax parsing passed.
• Glade XML parsing passed.
• Manually verified that Apply updates open terminals without closing
  Preferences.
• Manually verified that Apply can be used repeatedly.
• Close behavior remains unchanged.

Notes

The existing Discard Changes behavior is not modified by this pull request.
Its runtime rollback behavior appears to be a separate pre-existing issue.

AI assistance

Codex assisted with codebase inspection and implementation.

[mattrose]

This also looks good. TYSM