The setTargetLayerType(0) call in the ScriptedTool constructor happened
before the ScriptedTool instance was set as the prototype of the
provided script object. If the provided updateEnabledState would then
set the "this.enabled" property, it would override the Tool.enabled
property instead of changing it and cause all further attempts to change
the "this.enabled" property to not affect the Tool.enabled property.

Fixed by making sure the prototype is set before a possible call to
updateEnabledState happens.

Closes mapeditor#3756

The only known way to trigger this would be to implement a scripted tool
that sets "this.enabled" to false in the "activated" callback.

The argument passed to registerTool is not entirely the same as the Tool
instance it returns. Most of its properties are optional and some
properties are only available on the Tool instance.

The functions in ToolDefinition now define the type of 'this' as Tool.

See mapeditor#3756

Needed because otherwise TS assumes the references will never be null.