[DO NOT PULL] Multiline selection and copy

The work here is not finished. 
Not all the features are implemented, the code is ugly and I have my considerations about performance.

What works:
Multiline selection.
Copy to clipboard of said selection.

Todo:
Selection by word on double click.
Make selection stylable.
Column selection(?)
Check performance.
some other features? 

I created this pull request because I need some input from you guys. 
I don't really have any idea how to make double click work. 
I don't know if we need column selection at all. 
And I'm not good enough at Vala (or C) to make good judgments about performance.