Additional actions on helm-etags items and keybindings

This commit generalizes the `helm-etags-default-action' to accept a
function that is used on the filename.

This allows the addition of actions like find in other window/frame.
It also adds `C-c o` to open tag in other window and `C-c C-o` to open
tag in other frame.
Merge pull request #465 from markus1189/helm-etags-other

Additional actions on helm-etags items and keybindings
* helm-buffers.el (helm-buffers-persistent-kill): Fix preselection.