Add zoom control to View, add reset zoom functionality, closes #282

Zooming via Ctrl + NumpadPlus and Ctrl + NumpadMinus as well as Ctrl + MouseWheel was already supported. 

This adds zoom controls to the View Menu, support for the Ctrl + Plus and Ctrl + Minus shortcuts plus a new reset zoom functionality.

![](http://i.imgur.com/DCID4id.png)

The only thing missing right now is allowing Ctrl + Numpad0 to work as well for resetting the zoom, unfortunately `ToolStripMenuItem`s only support one `shortcutKey` it seems. Not sure what the best approach here is, I tried multiple things. An `OnKeyDown` listener in `MainForm` for example doesn't seem to fire while scintilla has focus.