ui: Add reusable classic-style popup foundation

[DevOpsOfChaos]

Summary

This extends the existing iwMsgbox implementation into a reusable classic-style popup foundation.

The goal is to support future warning/confirmation/info dialogs without adding one-off popup implementations.

This PR adds:

• configurable message box buttons
• custom button labels
• explicit result mapping
• default button handling for Enter
• cancel result handling for Escape/custom close
• optional message box icons
• visible internal demo popups in dskTest
• UI regression coverage for custom buttons and Escape handling

Motivation

Some future UI flows need stronger user-facing warnings, confirmations, or information dialogs. Examples include advanced addon settings that may affect gameplay heavily, map validation warnings, informational map details, and small accept/cancel confirmation flows.

Instead of adding a special-case warning popup for one addon, this PR makes the existing message box more reusable while preserving the classic in-game UI style.

This keeps the foundation generic so future dialog use cases do not need separate one-off popup implementations.

Related future use cases

This PR does not close any of these issues by itself, but it provides reusable groundwork for UI flows that are related to existing requests or discussions:

• [Feature] Copy links / IPs #107: could use a small accept/cancel confirmation dialog for copied links or IPs
• Maps silently not shown/loaded #1243: discusses showing a warning/message when maps are silently not shown or loaded
• Map Selection: Show ore distribution of map #1334: suggests an information dialog for additional map details
• Game crashes when S2 gamefiles not found #1788: shows that clearer startup/file-missing error messaging is useful, even though that specific case may not use iwMsgbox directly
• Alter the cancel dialog in the addon settings dialog box #767: shows that addon/settings dialog UX has already been discussed before
• Can't close addon window with rightclick #1471 and Create Game / Error windows can't be closed by right clicking #526: are related to dialog/window close behavior and cancellation consistency

The intent here is only to provide a reusable foundation, not to implement those specific flows in this PR.

Implementation details

This PR extends the existing iwMsgbox instead of introducing a parallel popup system.

It reuses existing classic UI elements:

• IngameWindow
• existing message box background/resource styling
• existing question/warning icons
• ctrlMultiline
• classic text buttons

New configuration types:

• MsgboxConfig
• MsgboxButtonConfig

Supported behavior:

• 1 to 3 custom buttons
• custom button labels
• custom MsgboxResult values
• configurable default button
• configurable cancel result
• optional icon
• Escape handling for custom-close message boxes

WindowManager was adjusted so custom-close windows can handle Escape through their own close/cancel behavior instead of being forcibly closed.

Demo / screenshots

This PR adds internal demo buttons to dskTest:

• Warning popup
• Confirm popup

Manual demo path:

1. Run s25client
2. Open the internal test screen by double-clicking the version text in the main menu
3. Click Warning popup or Confirm popup

Example usage

WINDOWMANAGER.Show(std::make_unique<iwMsgbox>(
  "Confirm", "Do you want to continue?", this,
  MsgboxConfig{
    {{"Yes", MsgboxResult::Yes, TextureColor::Green2},
     {"No", MsgboxResult::No, TextureColor::Red1}},
    0, // Enter/default button
    1  // Escape/cancel button
  },
  MsgboxIcon::QuestionRed, msgboxId));