fix: prevent segfault due to callbacks into deleted objects

Since network requests are, for the most part, asynchronous, there's a
chance a request only comes through after the request sender has already
been deleted.

This adds a global (read static) hash table relating models for the mod
downloader to their status (true = alive, false = destroyed). It is a
bit of a hack, but I couldn't come up with a better way of doing this.

To reproduce the issue before this commit: scroll really quickly through
CF mods, to trigger network requests for their versions and description.
Then, in the middle of it close the mod downloader. Sometimes this will
create a crash.

Signed-off-by: flow <flowlnlnln@gmail.com>

launcher/ui/pages/modplatform/ModModel.h

@@ -18,7 +18,7 @@ class ListModel : public QAbstractListModel {
 
    public:
     ListModel(ModPage* parent);
-    ~ListModel() override = default;
+    ~ListModel() override;
 
     inline auto rowCount(const QModelIndex& parent) const -> int override { return modpacks.size(); };
     inline auto columnCount(const QModelIndex& parent) const -> int override { return 1; };