First v7 beta

This is the first beta version for the new major version 7. It merges the v7 branch to master and in addition has a few fixes over the last alpha v7.0.0-alpha.4:

• Ubuntu build: added libopengl-dev dependency. Fixes #1244
• USRP enumeration: use default name and serial if not available. Fixes #1239
• Fixed device enumerator in-use indicator not reset on device set delete (solves issue reported in groups.io)