Remove dependence on gtkmm and libsigc++.

These dependencies have been replaced with thin, custom templated
wrappers for just the features I'm currently using.  Both the method
interfaces and memory management is handled in the new gtk.h file.

The uri entry struct is now a derived GTK class.  This allows
overriding of GTK virtual functions and signal registeration which
would not otherwise be possible.

Exception handling has been disabled because it is not safe to let
these begin unwinding GTK C functions, and currently any thrown
errors (e.g. std::vector out-of-bounds errors) are fatal.

RTTI has been turned off because I am not using C++ virtual
inheritence, dynamic_cast<>, typeid, exceptions, or any similar
features.  The behavior of All polymorphic GTK wrapper types is
determined at compile time.

Clean build times are roughly twice as fast now, and an -O3 binary
compiled with clang++ 3.4 is 100kB smaller.