API: Return URLs in escaped (percent-encoded) form

API: Return URLs in escaped (percent-encoded) form

- Extend `l_chk_s' and `l_chk_assign_s' functions to allow the caller
  set the `escape_url' arg to TRUE to tell the library to store the
  string value in URI escaped form

- All URLs returned by the library (e.g. media stream or thumbnail) are
  now stored in their escaped form, and passed as such to the
  applications querying them

- "Reserved chars" remain unescaped
  - https://tools.ietf.org/html/rfc3986#section-2.2

Affected API functions

- quvi_subtitle_lang_get (QUVI_SUBTITLE_LANG_PROPERTY_URL)
- quvi_playlist_get (QUVI_PLAYLIST_*_URL)
- quvi_media_get (QUVI_MEDIA_*_URL)

Rationale

- Address the potential problem of chars getting mangled when the URLs
  containing UTF8 chars are passed to the application trying to access
  the URL

Signed-off-by: Toni Gundogdu <legatvs@gmail.com>