Removed valueHint from effect parameter and turn all values form QVarian...

...t into double

While testing the Goaslicer https://github.com/mixxxdj/mixxx/pull/337
it turns out, that all other value hints, other than VALUE_FLOAT did not work.

This is due to the fact, that a knob gets out of sync if it is turned to other values than full integer in case of an integer value.

If an effect needs integer or boolean parameter, the can expressed as a double as well. The quantization and rounding should be done at the effect, if required. 

The feature of the QVariant to be invalid was not working, since it defaults to a double value range of 0.0 ... 1.0 anyway. These are the new defaults if no value is explicit set in the manifest.

This pull request cleans up the code and the effect interface a bit. The performance improvement should be only marginal.