Cleanup root directory

Cleans up the root directory of the GitHub source tree and starts to separate platform-specific installing and packaging logic from the master CMakeLists.txt.  Closes #2201

INSTALL.txt

@@ -0,0 +1,48 @@
+Building LMMS got quite simple since 0.4.0 as everything is managed
+by cmake now. Therefore make sure you have CMake (>= 2.6.0 recommended) and
+then run
+
+
+mkdir build
+cd build
+cmake ../
+make
+sudo make install
+
+If your system does not have "sudo", become root with your preferred mechanism
+and run the "make install" command.
+
+With the above commands an out-of-tree build is performed. You can also run
+"cmake ." directly in the root of source tree although this is not recommended.
+When performing an out-of-tree build after there's already an in-tree build,
+make sure to run "make distclean" before running cmake inside build-directory.
+
+If you want to use custom compiler flags simply set the environment variables
+CFLAGS and CXXFLAGS.
+
+After running cmake (the 3rd command above) you can see a summary of things
+that are going to be built into LMMS or built as plugins. Install the
+according libraries and development files if a certain feature is not enabled.
+Then remove CMakeCache.txt and run cmake again.
+
+If you want to supply an install prefix to cmake, add the flag:
+
+-DCMAKE_INSTALL_PREFIX=<prefix>
+
+Where <prefix> can be /usr, /usr/local, /opt, etc. The default is /usr/local.
+
+
+
+Building with QT5
+
+First of all please note that CMake >= 2.8.11 is required for building with
+Qt5 support. In order to build LMMS with Qt5, add the following flag when
+invoking cmake:
+
+-DWANT_QT5=ON
+
+If your Qt5 installation does not reside in standard installation paths,
+additionally pass e.g.
+
+-DCMAKE_PREFIX_PATH=/opt/qt53/
+