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.
Instead of creating new methods to MainForm, you could use directly the ScintillaCommand calls. You can create ResetZoom method to Scintilla.