Main menu activity layout finetuning

what is done:
* add icons
* remove background pattern (discussed with designteam)
* remove unused colors
* icons, arrow and text centered vertically
* adapt text colors
* adapt textsize
* adapt padding/margin: left to icon, richt to arrow, padding between icon

what is missing: 
* a 9patch drawable of button background (the background is still build in xml) - if I use the pngs, the black margin between continue- and new button is larger than between other buttons
* padding between continue text and projectname
* new continue icon from designteam (has a small artifact on the bottom)

testrun: http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test/480/

link to mockup: https://raw.github.com/Catrobat/DesignAndUsability/master/catroid/___Vermassung/01_startscreen_V.jpg

![main_galaxy_nexus_scaled](https://f.cloud.github.com/assets/1747262/243441/1ac2c89a-8a47-11e2-8909-5a747ddee7e3.png)