Adding support to persist views each frame.

* Added "dump_image" attribute to channels
* The corresponding evn variable is EQ_CHANNEL_IATTR_DUMP_IMAGE
* Added default implementation of FrameWriter that writes to a file
* Some refactorings in order to make the code a bit more readable

Compiled and tests.

Ran
> seqPly
> seqPly --eq-config <and non existing session>
> seqPly --eq-config good.eqc ->
    once using channel attribute on
    once using global attribute on
    once without dump image attribute anywhere, but env variable defined

all working as expected.