linput: Rewrite input parsing completely

* FIX (memory leak): Release the GIOChannel created for the stdin
  by decrementing the reference count of the <stdin> GIOChannel

* Do not unescape input URLs
  - This was previously done because quvi-dump shamelessly escaped the
    URLs completely (incl. the "reserved chars") and g_uri_parse_scheme,
    which linput calls (to determine the action it should take with the
    input URL), couldn't parse the escaped URI schemes otherwise

  - Any normal applications leave the "reserved chars" unescaped, e.g.
    the web browsers

* linput_new now returns an integer, indicating the success
  - Program now exits if input parsing failed (e.g. invalid URI)

* Follow file URIs
  - Making it possible to pass "file://" URIs to the program

Signed-off-by: Toni Gundogdu <legatvs@gmail.com>
lutil_parse_input: Check linput_new return value

Signed-off-by: Toni Gundogdu <legatvs@gmail.com>
DOC: input: Add file URI note

Signed-off-by: Toni Gundogdu <legatvs@gmail.com>
Merge branch 'tg/next__redo_linput' into next