plasma-framework: Convert copyright statements to SPDX expressions

Convert copyright statements to SPDX expressions and introduce license texts according to REUSE specification.

Note that for a few source files that originate from the Qt5 technical preview, I removed the alternative license option to use the file under the terms for the technical preview licenses (which I could not obtain), because the LGPL-2.1 license is enough. Moreover, this already was done this way for other files in the repository.