"Reset to Center" and "Zoom to Drawing" Functions

[tafode]

As described in #233 I have created two new buttons in the toolbar. One for resetting the camera to center/home (position 0/0 with Zoom 1) and one that zooms out so that the whole drawing is shown.

I'm not sure if the Icons are clear enough. So I'm open for suggestions. Additionally they don't match exactly the style of the others.

Known issues with this solution:

• If it would be necessary to zoom out further than 100 to see the whole drawing, it stops at the limit of 100 and only shows a part of the drawing.
• In some cases, if the window is very small, a small part of the drawing may be hidden under the toolbar when zoom to drawing.