Add SQ code support

[jasp00]

This patch adds support for SQ code, a simple 2D barcode.

[mchehab]

Thanks for the pull request.

The patches themself look good, although I was unable to test the SQ code parser.

A few comments:

1. please add a description to each patch. That makes easier to identify what they're doing and helps a lot when looking at regressions, as it allows to identify why a certain change was needed;
2. I'm adopting the practice of adding a certificate of origin of a patch, the same way as we use at Kernel trees (see https://linuxtv.org/wiki/index.php/Development:_Submitting_Patches#Developer.27s_Certificate_of_Origin_1.1).
   So, please add your SOB to the patches.
3. While you're here adding a new 2D barcode, it would be cool if you could also update the helper message when a barcode is not found. currently, it displays:

- is the barcode type supported?  currently supported symbologies are:
  EAN/UPC (EAN-13, EAN-8, EAN-2, EAN-5, UPC-A, UPC-E,
  ISBN-10, ISBN-13), Code 128, Code 93, Code 39, Codabar,
  DataBar, DataBar Expanded, and Interleaved 2 of 5

(yeah, I know, it also doesn't mention QR... feel free to add it as well)

4. are there any SQ code example that we could use to test the parser? On a quick search, I was unable to find one that identifies itself as SQ. If you can generate one (or find an image that it is compatible with Zbar license), it would be cool if it could be added into the examples/ directory. Otherwise, please provide us an hiperlink, in order to help testing it.

[jasp00]

Done.

[mchehab]

See issue #8
There's something broken with SQ code. Trying to parse another barcode format causes it to loop forever.

While this is not solved, I'll disable sqcode from the default and add a warning.

[mchehab]

FYI, I got a hard to track bug with SQ code.

Please see:
6873d05

It is trigged when trying to read or set ZBAR_CFG_UNCERTAINTY for the SQ decoder. Only caught because zbarcam-qt now reads all configurations from all enabled decoders.