Introduce caps_converter function for format plugins

[dgollub]

This is a left over from #1084

• get a new format plugin function called "caps_converter"
  • osync_bool (* caps_converter) (OSyncCapabilities *old, OSyncCapabilities **new, OSyncError **error)

This function should allow to reuse other's format-plugin merge/demerge function.

Example:

• xmlformat plugin has a generic merge/demerge function
• evo2-format plugin just converts evo2-caps to xmlformat-caps and can use
  • so the evo2-sync plugin doesn't have to maintain a complete set of merge/demerge code
  • several evo2-format converts allow to support more formats then the xmlformat... but also other "common" formats

[dgollub]

Have a look at initial work from Björn in r5698

[dgollub]

(In [5750]) PoC of caps-converter in vformat plugin for capabilities conversion from plain vcard fieldnames to xmlformat(-contact)

refs #1146

[dgollub]

initial caps-converter implementation is complete