Fix MergeAll Toolbar Actions

In case of 2-way comparison, only left differences should be taken into
account, even when merging from right to left (all diffs are on the left).

The toolbar was initialized too late, and was thus not registered against
the event bus.

Change-Id: Ieaae914b9c71edb4566242b4ee8c377fadafacd7
Signed-off-by: Axel Richard <axel.richard@obeo.fr>