replace long license header text with SPDX identifier

The license texts are tricky to read. One word changed and you don't know what the license stands for anymore.

Lately there has been a trend of using a simple identifier in the source code to make it easier to read and clearer what the license of a file is. More info on this can be found at https://spdx.dev/

This MR changes most of the LGPL/GPL/BSD/MIT code we have. Each commit mentions the code it replaces.

Some files have not been modified as they have dual license (not sure how it works in this case).