Add init interface with main_window as a parameter

Add init interface with main_window as a parameter

Since libscl-core creates input panel window automatically
by itself, it was not possible to create a window outside
of libscl-core and use it as main input panel window.
So added a new init interface that accepts window as a
parameter which will let libscl-core to use it as a main
input panel window. When NULL given or this new init function
is not called from outside, libscl-core will create input panel
window by itself when starts running, as before.

Change-Id: I001edc342f1090ed33018eab04e8a90e55d60d21