Glade Plugin improvements

Overall

Make the plugin a bit smoother and fix some small issues:

• Glade menu is cluttered - a preference page would be better
• The Devhelp button doesn't really work

Signals

• Add a signal tree to glade (or expose that somehow)

• Adding a signal to a source file should be done using drag&drop

  • User drags signal from the signal tree to the editor
  • Depeding on the language the appropriate language-support-plugin decides what to do:
  • For C:
    • Header: Insert prototype at drop position
    • Source: Insert implementation at drop position
    • Warn the user if the symbol is already defined in the file
  • For C++: Nothing, as C++ doesn't support signal auto-connection
  • Python, Vala, etc...