Git commit infos on Doxygen front page and Data Explorer About dialog

This introduces two evil :rage: global variables (see this [StackOverflow answer](http://stackoverflow.com/a/4318642/80480) why this is good) which hold the current git commit hash. Usage:

```c++
#include "GitSHA1.h"
...
cout << "Short: " << g_GIT_SHA1_SHORT << ", long: " << g_GIT_SHA1 << endl;
```

Make sure to link with `BaseLib`.

This is used in the Data Exlorers About Dialog.

There are also two corresponding CMake variables:

- `GIT_SHA1`
- `GIT_SHA1_SHORT`

They are used to show the commit hash on the generated doxygen front page.