Fix and normalize license in .desktop files
Summary:
The license is often specificed as (L)GPL, which means v2, while the
actual code is usually v2 or v2+. This patch reflects that.
Test Plan: checked source files, compared with kaboutdata.cpp keys
Reviewers: Plasma, sitter, mart, broulik
Reviewed By: Plasma, sitter, mart, broulik
Subscribers: mak, mart, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D6628