Merge AbstractClient and Toplevel

Currently, we have the Toplevel class and the AbstractClient class. The line between them is very blurry. There were multiple cases when we lifted things from AbstractClient to Toplevel, just to simplify code. It would be great if the two are merged.

Unfortunately, we can't just merge them as there's code such as

auto foo = qobject_cast<AbstractClient *>(toplevel);
if (foo)
    foo->doSomething();

The first step would be to replace the qobject_cast<>()s with the following logic

if (!toplevel->isClient())
    return;
auto foo = static_cast<AbstractClient *>(toplevel);

In the second phase, do the actual merge.

Here's a draft strategy (each point can be a separate MR):

1. Change the type checks as described above for AbstractClient (✅ !2224 (merged))
2. Change the type checks also for Deleted and Unmanaged (✅ !2232 (merged))
3. Inherit both Deleted and Unmanaged from AbstractClient (✅ !2236 (merged)) (implies overriding virtual functions)
4. Move properties of Toplevel into AbstractClient and… (✅ !2250 (merged))
5. …remove Toplevel class (✅ !2250 (merged))
6. Rename AbstractClient to Window (✅ !2257 (merged))
7. Janitorial taks (✅ !2284 (merged), !2282 (merged))