Refactor build scripts to build Urho3D library as main target.

Other changes in the commit:
- Change build option from URHO3D_BUILD_TYPE to URHO3D_LIB_TYPE. Default value is STATIC.
- Automatically add compiler definition 'URHO3D_STATIC_DEFINE' when linking with Urho3D library statically.
- Move LuaScript subsystem from Extra/ to Engine/.
- Enable FindUrho3D module to also detect Urho3D library target when the FindUrho3D module is being used internally by Urho3D project.
- Configure to build all the samples for iOS and Android platforms also when option to build samples is enabled.
- Export all the Register*Library() global functions.
- Rewrite all the tools, samples, and extras targets to link against Urho3D library.

TODOs:
- Combine engine sub-libraries into one.
- Test builds on non-Linux platforms.