mouse cursor management

[igagis]

unify mouse cursor manager to allow setting own mouse cursor manager in case of using for example image_mouse_cursor widget to draw mouse cursor.

[igagis]

need to add set_mouse_cursor_handler to ruis::render::native_window

[igagis]

note, that capturing shared_ptr to a widget in the set_mouse_cursor_handler lambda will cause circular reference:

ruis::widget -> ruis::context -> ruis::render::context -> ruis::render::native_window -> ruis::widget