CMake improvements

• CMake will output binaries to CMAKE_BINARY_DIR/bin
• The SRB2_COMP_REVISION constant will be configured with not only the contents of git describe but also the current branch name appended to the end.

Normally, compilation would toss the binaries in the build-side source directories for each target (src/sdl for sdl2, src/win32 for dd). This makes all targets output to a single directory.

In addition, source files are split into groups, and core code is separated into its own target apart from interfaces.