Added word size menu button to programming mode

I've wanted this feature for a while, so I thought why not write it myself?

Added a GtkMenuButton to the free spot in the upper-right hand corner of programming mode. Then added the code that handles changing the word size, and updating the label to reflect the current word size.

This is my first time contributing to GNOME, so please let me know if I've made any errors. I'm also far from proficient with Vala, so a lot of what I was doing I was basing off of other code in the same files. I think I have followed the code style correctly.

Fixed the issue with the button label not tracking the state of equation.word_size.