Add aglwgt class

Add aglwgt class

This change adds a aglwgt.bbclass which does:
- simplify the application (wrapper) recipes within AGL
- enforces that 'make package' is present to guarantee the SDK workflow works
- places the wgt in the standard location of the AGL reference distro /usr/AGL/apps/

Later the following features will be added to the class:
- automatic generation of a wrapper for smoke-test of install/uninstall operations (ptest script)
- build-time format checks

Change-Id: Ie4f8281061febf3495d33bb1483f8c64b54ed8e9
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Signed-off-by: Stephane Desneux <stephane.desneux@iot.bzh>