SPDX license migration

Move the various licenses found in Krita's files to the SPDX format.

licensedigger has been used using the following options licensedigger --ignore "CMake|3rdparty|.cmake".

Note 1: there are files coming directly from Qt's sources that might be interesting to update so they have not yet been migrated

Note 2: the changes done to licensedigger have not yet been integrated to the tool

Note 3: licensedigger does not yet contain the full set of licenses used in Krita but a merge request is pending approval.