Make occur-x a minor mode.

Change-Id: I59ea6c664bae9c7a426939af2159ffc14df04587
Make hooks local, since occur-x is non-global

Change-Id: I2bb8343f675545c40ede5e7f04306fa5fd33c1ea
Avoid losing filters when going into occur-edit-mode

Change-Id: I856fc8ab6fb6fb9b1b6a26b59da3a4622f38494f