Updated world scene look

World scene's control bar now has a look inspired by Holo Light. Added
menu and back button to control bar that behave the same way as the
Android buttons. Button class now supports tinting (and transparency
blending). Button class modified to support buttons without text and
buttons with dynamic behavior and text. Avatar walks slower and
animation is sped up. Draw a single avatar in world scene instead of 192
of them. Draw grass (texture by Gregory Johnson) on all 192 tiles in
world scene. net.pjtb.celdroids.client.android.mainmenu.wifidirect
package renamed to net.pjtb.celdroids.client.android.menu.wifidirect.
Allow game to be run with software rendering on desktop PCs that don't
support hardware accelerated graphics.