Rework signal_connection_t

[ammen99]

signal_connection_t and signals as a whole turns out quite unpleasant to work with in language bindings (wflua being an example, my own attempts at C bindings as well). We also have the signal names spread across many files, and completely unnecessary type casts everywhere. A better design for signals would be:

• signal_provider_t emits the signal together with:
  1. this
  2. Signal name
  3. Untyped void* data, it is no worse than signal_data_t*, the latter doesn't have RTTI anyway since it doesn't declare any virtual methods.
• signal_connection_t works as it does now, with the additional parameters from above
• A new helpers for typed signals which are 99% of all signals:
  • Helpers use templates to specialize to various signal types
  • Signal name is either signal_type_t::name or as a fallback the typeid.
  • Signal handler also knows the type (so no need to specify the signal name), and auto-casts the signal data to the correct type.