Remove distinction of widget's version

This removes the distinction between the different
versions of a widget. The boolean CMAKE option
DISTINCT_VERSIONS allows to switch between the two
possibilities:

 DISTINCT_VERSIONS=OFF (default)

    Widget installed in {afm_appdir}/{id}
    Ids of applications have no version part: {id}

 DISTINCT_VERSIONS=ON (legacy, old default)

    Widget installed in {afm_appdir}/{id}/{ver}
    Ids of applications have no version part: {id}@{ver}

Bug-AGL: SPEC-2538

Change-Id: I7cb54d4b296b740c553be8a627e66175107e5a4b
Signed-off-by: Jose Bollo <jose.bollo@iot.bzh>