API: implemented POST and DELETE /sdrangel/featureset
This API allows creation and deletion of feature sets. This fixes #988 and makes it possible to work with features in the server variant. Also fixed GET /sdrangel/featuresets.
Multiple feature sets have been implemented in the GUI. There is no special affinity between features in the same feature sets unlike device sets for which one device and one to many channels share the same I/Q stream. This is rather a way for the user to organize features in logical groups that are displayed together in the same feature tab.
Binary artifacts
For more details on artifacts and their contents please read the quick start in the Wiki.
Note that Ubuntu .deb package is for 20.04. Please note that since v6.2.0 SSE 4.2 is required. It has been intended to be like that for a long time but was not effective until this release.