Implement Out-of-Source Build

Implements an out-of-source build strategy for the GUI code, where
the GUI will be built in a build directory, leaving the source tree
clean.

Changes:

 * src/GUI/build.sh: Script that performs the out-of-source build.
 * src/GUI/CMakeLists.txt: Copy python files needed to run perftrack
   to the build directory.
 * install.sh: Modify to use build.sh to do the actual build.
Implement Make Install For GUI

Modifies the CMake config to add "make install". The perftrack GUI is
installed in ${CMAKE_INSTALL_PREFIX}/bin, and the python plugins are
installed in ${CMAKE_INSTALL_PREFIX}/lib/perftrack-plugins.

Changes:

 * src/GUI/CMakeLists.txt