Make FossilisationDialog a CustomWindow

[IdfbAn]

Brief Description of What This PR Does

This PR changes FossilisationDialog so that it's derived from CustomWindow rather than CustomConfirmationDialog. Nothing seems to have broken even though I changed literally only one line.

Related Issues

Closes #4157

Progress Checklist

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• PR author has checked that this PR works as intended and doesn't
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  (this is important as to not waste the time of Thrive team
  members reviewing this PR)
• Initial code review passed (this and further items should not be checked by the PR author)
• Functionality is confirmed working by another person (see above checklist link)
• Final code review is passed and code conforms to the
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[hhyyrylainen]

This doesn't actually do anything...

[hhyyrylainen]

I think you didn't change the uid so this change does absolutely nothing, at least Godot Editor still shows it inheriting the confirmation dialog to me...

[hhyyrylainen]

Sure enough when I fix that, things actually start breaking so this PR still needs work (I'll commit my uid fix to this):

E 0:00:12:610   int System.Runtime.CompilerServices.CastHelpers.<IsInstanceOf_NoCacheLookup>g____PInvoke|4_0(System.Void*, int, System.Runtime.CompilerServices.ObjectHandleOnStack): System.InvalidCastException: Unable to cast object of type 'CustomWindow' to type 'CustomConfirmationDialog'.
  <C++ virhe>   System.InvalidCastException
  <C++ Lähde>   :0 @ int System.Runtime.CompilerServices.CastHelpers.<IsInstanceOf_NoCacheLookup>g____PInvoke|4_0(System.Void*, int, System.Runtime.CompilerServices.ObjectHandleOnStack)
  <Pinojäljitys>:0 @ int System.Runtime.CompilerServices.CastHelpers.<IsInstanceOf_NoCacheLookup>g____PInvoke|4_0(System.Void*, int, System.Runtime.CompilerServices.ObjectHandleOnStack)
                :0 @ int System.Runtime.CompilerServices.CastHelpers.<IsInstanceOf_NoCacheLookup>g____PInvoke|4_0(System.Void*, int, System.Runtime.CompilerServices.ObjectHandleOnStack)
                :0 @ object System.Runtime.CompilerServices.CastHelpers.ChkCastAny_NoCacheLookup(System.Void*, object)
                VariantUtils.generic.cs:398 @ T Godot.NativeInterop.VariantUtils.ConvertTo<T>(Godot.NativeInterop.godot_variant&)
                FossilisationDialog_ScriptProperties.generated.cs:61 @ bool FossilisationDialog.SetGodotClassPropertyValue(Godot.NativeInterop.godot_string_name&, Godot.NativeInterop.godot_variant&)
                CSharpInstanceBridge.cs:57 @ Godot.NativeInterop.godot_bool Godot.Bridge.CSharpInstanceBridge.Set(nint, Godot.NativeInterop.godot_string_name*, Godot.NativeInterop.godot_variant*)