Add support for a system-wide installation

[s3rvac]

RetDec should be installable directly into the system. Among other things, this will require giving more unique names to our tools (#70).

[s3rvac]

After merge commit e7d15c9, RetDec is installable directly into the system. During the installation, the following structure is created:

• bin/ (contains e.g. retdec-decompiler.sh, retdec-fileinfo)
• lib/ (on Linux and macOS, it contains libretdec-libdwarf.so and libretdec-libelf.so)
• share/retdec (contains e.g. doc and support)

Nevertheless, even though RetDec now supports a system-wide installation, unless you use your distribution's package manager to install it, we recommend installing RetDec locally into a designated directory. The reason for this is that uninstallation will be easier as you will only need to remove a single directory. To perform a local installation, run cmake with the -DCMAKE_INSTALL_PREFIX=<path> parameter, where <path> is directory into which RetDec will be installed (e.g. $HOME/projects/retdec-install on Linux and macOS, and C:\projects\retdec-install on Windows).