Reduce convert from std::string to VisualUrl

Reduce convert from std::string to VisualUrl

Whenever we create new VisualUrl from raw-string, it might calculate
the hash, or protocol check one more time.

To avoid this thing, let's make we use VisualUrl where we can use it.

Note : Since ImageUrl can only use std::string itself instead of VisualUrl,
we should make some API to remove external textures by std::string.

Change-Id: I6888f60e6060aae79d7d11dbc7cf0aa4004d4266
Signed-off-by: Eunki, Hong <eunkiki.hong@samsung.com>