Unified UI for downloads

[PiKeyAr]

This is a feature request based on the concepts I implemented in the WinForms-based SADX Mod Manager.

The idea is to have a single window for everything related to downloads and updates, including mod updates, mod downloads from 1-click installation, Visual C++ runtimes (in my implementation there was also an option to add DirectX 9.0c on first run), updates for the Loader, extlib dependencies, Manager and Launcher. All download and installation processes would happen when this window is displayed, without pop-ups in the Manager UI.

Additional features to consider:

1. Checkboxes to let the user decide which updates to install.
2. Info about downloads: changelog, GitHub or GameBanana info etc. Basically a combination of the mod update dialog and the 1-click install dialog.
3. Manager updates are always listed last to ensure all other items scheduled for update are not discarded.
4. Verbose download statistics for current item and total progress + progress bars.

The screenshots below demonstrate the WinForms implementation.

[PiKeyAr]

Implementation: the class used for mod download metadata could be expanded to include a "download type" value (Mod, Launcher, Manager, VC runtime etc.), and each download type would have a separate function that is called after the download finishes.