Add API to check wheter the given encoding is supported by WebKit

Add API to check wheter the given encoding is supported by WebKit

[Title] Add API to check whether the given encoding is supported by WebKit
[Issue] N/A
[Problem] The client is able to set encoding even if WebKit does not support it.
[Solution] Expose ewk_settings_is_encoding_valid(const char*) API to check
           whether WebKit supports the requested encoding.
           This checking could be done while setting the encoding in
           ewk_settings_default_encoding_set. However, WRT Team prefers
           separated API.

Change-Id: Ib38fc266082d30c35a0716ee859894bd7fd702a1