Add support for webp image format once integrated into wxWidgets 3.3

[KeyWorksRW]

Description:

Support for the webp image format is being added to wxWidgets, however the actual source code is not included. Linux systems may have the library as a system package, but you still have to install the libwebp-dev to get the headers. For Windows, you have to have vcpkg installed.

The other alternative is to get the source code for webp and build it as part of wxUiEditor. Source code is at https://chromium.googlesource.com/webm/libwebp -- which means we can't use this a submodule because it's not a github repository. We could copy specific files from the latest release tag (currently 1.4). We'd still have to tweak our copy of wxWidgets since the header files would not be where wxWidgets expects them.

The advantage of this approach is that it should work on Linux and Windows without requiring the user to install a package. That would allow wxUiEditor to preview the webp images -- the user will still have to install the library and build their own copy of wxWidgets to actually use them.