added GLKeysES30 sample

 - Android and iOS, includes shared code, which uses opentk library
use mipmaps to speedup rendering
load shaders source code from EmbeddedResource

 - this way we can share shaders loading code between Android and iOS
 - thanks to Jonathan Pryor for suggesting that
use GetErrorCode instead of GetError

 - GetErrorCode is preffered as it can be shared between iOS and Android
 - GetError is now obsolete
added TexturedCubeES30 sample

 - it is based on monodroid-samples/TexturedCubeES30-1.0 and
   contains iOS app as well
remove casts

 - OpenTK's ES 3.0 enums were extended and thus we don't need these cast
   anymore
use Shared project to share .cs sources between Android and iOS
use shared project for shaders source as well
updated to use non-obsolete methods with strongly typed enums
updated to use methods with strongly typed enums

 - instead of obsolete ones
limit xDiff, yDiff to avoid too fast rotation
re-enable fast deployment + shared runtime for debug builds