Make OCR skip options configurable

[R0Wi]

As a result of #113 we should make the options --skip-text, --redo-ocr and --force-ocr configurable through our per workflow settings UI. Also we should have an option so choose none of these parameters (see #130)

• Add a flag to the model
• Add radiobuttons to per workflow settings UI
• Make redo-ocr mutially exclusive with remove-background (latter is not possible in redo-ocr-mode)

[jkumeboshi]

With skip-text as default, documents created with camscanner or other similar SW adding some text to the scanned images fails ("scanned with CamScanner" is enough to stop the skip-text option).

I think that this setting could be added to the workflow settings.
So I would create 2 kind of tag forkflow: 1 with redo-ocr or force-ocr and another with skip-text
So I could use the force-ocr workflow with documents coming from camscanner

[R0Wi]

@jkumeboshi I implemented the configuration possibility now. Could you image beta testing the new feature and give me some feedback?