Add project properties support

[pblottiere]

This PR adds a generic method to handle all QGIS project properties based on readEntry and writeEntry methods. For now only WMSAddWktGeometry and WMSPrecision are registered (to fix #70), but more can be easily added in the future (PAL, service capabilities, WMS, ...).

For example to retrieve all properties for a specific project:

$ curl "http://localhost:5000/api/projects/my_project/properties"
{
  "wms": {
    "getfeatureinfo_geometry": false,
    "getfeatureinfo_geometry_precision": 8
  }
}

Then to update specific properties:

curl "http://localhost:5000/api/projects/my_project/properties" \
  -X POST \
  -H 'Content-Type: application/json' \
  -d '{
    "wms": {
      "getfeatureinfo_geometry": true
    }
  }'

[pblottiere]

Hello @JakobMiksch,

What do you think about this PR?

[JakobMiksch]

elegant idea. this would make #75 superfluous as well I guess.

regarding the naming. This PR uses the term getfeatureinfo_geometry and translates it to WMSAddWktGeometry. This is indeed more userfriendly as API user, but a lot of work to implement. I am just wondering, if the API user could instantly edit the QGIS Projects using the terms used in the QGIS project file directly.