Optimization to reduce Handle size by 50%

Optimization to reduce Handle size by 50%

[problem]      Memory usage is too high, event-thread operations are too heavy.
[cause]        Too much Dali::Handle data copied around.
[solution]     Remove all virtual methods from handles, to avoid needing a C++ vptr

Change-Id: I8322c4ee56341b88c752da3931a7187c46eb9419
Signed-off-by: Adeel Kazmi <adeel.kazmi@samsung.com>