Incapsulate of CK_FileNext/CK_FilePrev command handling inside mcviewer.

[andrew_b]

Currently, when CK_FileNext/CK_FilePrev command is raised in mcviewer, there following actions are performed:

1. close mcviewer
2. switch to file manager
3. run new mcviewer.

mcviewer is assign with current panel and its working dir. As a result of that, CK_FileNext/CK_FilePrev command handling doesn't work:

1. in mc which run as standalone viewer (mcview or mc -v)
2. with multi-screen engine:
   1. run internal viewer from file manager (press f3);
   2. switch back to the file manager;
   3. switch back to the opened viewer and press C-f (CK_FileNext) or C-b (CK_FilePrev);
   4. viewer is just closed.

The offering solution provides the full handling of CK_FileNext/CK_FilePrev commands within mcviewer and fixes problems described above.

Also this is a step to #2156.

Branch: 2814_viewer_next_prev (parent: master).
Initial changeset:ba8dcbe072928725db81a4b762fbcc7e26953548

[andrew_b]

Merged to master: [d235c71dae84d72a9bc39c4a20f8db2758f11ae4].

git log --pretty=oneline 50f8b5b..d235c71