Updating User Manual Screenshots to 2.0 - Introduction

This is my fist pull request for updating user manual screenshots to 2.0. I tried my best to follow the recomendations made by Otto Dassau and  Jean Roc Morreale.

Please check the following:
- I made a small change to the document "qgis_gui.rst" itself, since I used circles instead of ellipses in the "startup.png". (I don't know if i'm supposed to do that kind of small changes, or if I should inform the person in charge for that section);
- I added a new screenshot for grid decorations as it is a new feature in 2.0, and it might be needed;
- Since it would imply a very large mage, I gimped "panels_and_toolbars.png" a bit, cutting a peace of "unnecessary image" and replacing it for "...", I don't know if this is desirable and more important, clear enough.

If something is wrong, please let me know.

Best regards,

Alexandre Neto