Fix Exec line of desktop files.

They needed %f to indicate how to pass the file we're claiming to handle.

Update CMakeLists.txt

Version 3.0

Clarify ambiguous license terms.

The about box has always said "GPLv3 or later", but other references
failed to distinguish between "or later" and "only".

Improved de_DE

See solvespace#890 and its changed menu item

Version 3.0, release candidate 1

Version 2.3

Release 2.3 is a bug-fix release.

Version 2.2

Release 2.2 is a bug-fix release.

Version 2.1

Release 2.1 introduces *nix compatibility, internationalization,
technical drawing mode, rewritten DXF export, DXF import, and many other
minor features and bug fixes.

Version 2.1, release candidate 1

Release 2.1 introduces *nix compatibility, internationalization,
technical drawing mode, rewritten DXF export, and many other minor
features and bug fixes.