Fixed samples for the latest changes in Gradle plugin.

- Smaller gradle scripts.
- More structure in sample projects.
- Simplified build/run.
- Added gradle to the rest two samples.
- Almost removed all sh scripts.
- Gradle now works only inside of build directory.
- Fixed READMEs according to the new changes.

backend.native/tests/build.gradle

@@ -1833,13 +1833,13 @@ kotlinNativeInterop {
     }
 
     sockets {
-        defFile '../../samples/socket/sockets.def'
+        defFile '../../samples/socket/src/c_interop/sockets.def'
         flavor 'native'
     }
 
     if (isMac()) {
         opengl {
-            defFile '../../samples/opengl/opengl.def'
+            defFile '../../samples/opengl/src/c_interop/opengl.def'
             linkerOpts '-framework', 'OpenGL', '-framework', 'GLUT'
             flavor 'native'
         }