ST & WT: automatic prompt for "reference project" when trying to preview sounds

[JoeyQuint]

SoundTool and WadTool can both load a "reference project", in order to be able to play sounds according to said project's settings.

However, newcomers don't necessarily know about this concept: a common question is "how can I preview sounds in {toolname}"?

Idea

If no reference project is loaded:

• In SoundTool, do not freeze the "In-game preview" button.
• In WadTool, do not freeze the "Toggle Sound Preview" button. But switch sound preview off.

Then, when the user clicks this button, prompt them: "{toolname} requires a reference project to preview sounds. Do you want to load one now?".

• If the user hits No, just close the prompt and do nothing.
• If the user hits Yes, load the reference project.

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This way:

• Newcomers who do not know about this requirement and the concept of reference projects will discover it by themselves naturally
• Even for seasoned builders, it saves a few clicks - especially in WadTool, considering you'd have to close the animation editor and then reopen it and find the anim you were working on