[PATCH] Account for virtual plugins when offering to download and install missing plugins

[taylor.smock]

When updating Mapillary, javafx is not automatically installed. JOSM catches this on startup, but doesn't actually install javafx (since its a virtual plugin).

What this patch does:
1) Detects if a plugin can be installed on this platform
2) Detects if the plugin provides the "required" plugin
3) Detects if there are multiple candidates, and removes the possible duplicates (for safety (tm))
4) Performs the rest of the missing-plugin steps

What this patch does not do:

• Detect that a plugin was uninstalled previously (since the plugin isn't downloaded, it isn't added to the plugin list, which means that it is not loaded)

[taylor.smock]

Install virtual plugins, when they are missing

[taylor.smock]

Remove duplicates had a slight bug. remove if (required plugins don't contain the plugin name or provides) OR if the plugin is not for the current platform (might be redundant). Instead of remove if (required plugins don't contain the plugin name or provides) and is for the current platform.

[taylor.smock]

#KeepAlive (is this desired?)