ContentPackages are defined in mod.yaml and list Installers that support them, but then the Installers and their SourceActions knew nothing about ContentPackages.
Also added BeforeInstall and AfterInstall sections for SourceActions in the Installers.
- Add prefixes to all message keys to provide context
- Use messages with attributes for some UI elements (dropdowns, dialogs, checkboxes, menus)
- Rename some class fields for consistency with translation keys
When I downloaded the assets for Red Alert through the Quick Install I noticed the progress bar proceed and display a recognizable message: `Downloading from … 1.47/12 MB (12%)`. This was fine.
When I downloaded the assets for one of the other games, maybe Dune 2000, there was obviously no total download size available. I was an unexpected message: `Downloading from … 1.47/NaN (NaN%)`
The code handling network progress events seems to be aware of the possibility that no full download size exists but it doesn't update the message. In this path I'm proposing that we display a separate messaging indicating that we don't know how much more we have to download for these cases.
Of the alternative ways to implement this I chose to move the reassignment of `getStatusText` into the conditional structures to preserve the existing choice. The message was qualitatively different and so I felt it worthwhile to create entirely different closures vs. doing something like this…
```cs
getStatusText = () => ( Double.isNaN( dataTotal ) ? "Downloading {1} of unknown amount" : "Downloading {1}/{2}" ).F( … );
```
We were currently dealing with this terrible global variable in FileSystem/ZipFile.cs previously, but other parts of the code such as DownloadPackageLogic were creating these files too, and may not have executed the static ctors that fixed the encoding yet.
