Add support for automatic WebP -> PNG

[ovsky]

This pull request adds support for automatically converting downloaded WebP images to PNG format on Windows, improving compatibility for users who may not be able to view WebP files natively. The conversion is controlled by a new user setting, and the feature is integrated into the download management workflow. The most important changes are grouped below:

WebP to PNG Conversion Feature:

• Added a new setting convert_webp_to_png to UI::BrowserSettings and exposed it in the settings catalog, allowing users to enable or disable automatic WebP conversion. [1] [2]
• Implemented a Windows-specific function ConvertWebPToPNG in DownloadManager.cpp using Windows Imaging Component (WIC) to convert WebP files to PNG after download completion.
• Integrated the conversion workflow into DownloadManager::OnFinishDownload, updating the download record to point to the PNG file and removing the original WebP file if conversion is successful. [1] [2]

Download Manager Integration:

• Added SetWebPConversionCallback to DownloadManager to allow UI code to control whether conversion should occur, and stored the callback in the manager. [1] [2] [3]
• Connected the new callback in both UI constructors so that the user's setting is respected during downloads. [1] [2]

Other:

• Updated the fallback settings catalog size to accommodate the new setting.

These changes provide a seamless experience for users who prefer PNG images and ensure that downloaded images are compatible with a wider range of applications and viewers.