[DO NOT PULL] Issue #7 Selection and Copy

This is a second pull request on the same subject because I screwed up with my git branch and did not know how to undo it. I closed the previous one. 

This is my implementation of selection and copy. 

I made this PR because I am not sure that the way I implemented it is a good one.

It is not yet done and there probably are some bugs, the code is not up to standard and I have concerns about performance. 

I was doing it mostly for fun, so if it will be decided to proceed with this implementation I will be working on solving the problems mentioned above.

What works:
Normal selection
Word selection (double click)
Line selection (triple click)
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What needs to be done:
Stylable selection
Characters that are considered part of the word should be changeable via settings window (?)
Column selection (? @p-e-w wrote that we can do without it for the time. The bad thing about this implementation is that we will probably need to do some major changes to line creation and wrap system to make column selection work ) 

PS It will not be too hard to implement sublimetext-like multiple selections. Are there plans for this?
 
`on_text_container_motion_event` should also make use of this utility method to avoid code duplication.