lttng.h API update: set filter becomes enable event with filter

The lttng_set_event_filter() is changed to
lttng_enable_event_with_filter() taking the same arguments. The lttng UI
now only uses this call. Note that the original lttng_enable_event() is
still available but will set the filter to NULL.

This is done since now UST allows to enable the same event with
different filters or/and loglevels. So, the events are still hashed by
name but matched by the name/filter/loglevel triplet. In order to add an
event to the hash table, those three attributes are needed at the
creation time thus adding this API call which takes them all at once.

There is some fixes in the match functions and filter setting from the
previous commit that were needed to make the overlap.sh tests works.

The loglevel_match function is removed because it is now only done in
the hash table match function which will eventually get merged making a
single loglevel match call site hence this function becomes useless.

Furthermore, the filter.c/.h are no longer required since the filter is
now added at event creation and CAN NOT be set after.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>