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.

I am open for any suggestions/improvements, the main reason was that I really missed the 'C-c o' to open in other window as is already defined in helm-find-files and helm-buffer.

Best,
Markus