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