Add NSIS file for use with cross-compiled PulseView.
Replace GPLv2+ references with GPLv3+.

Most of the PulseView files may be licensed under the GPLv2+, but by linking
against libsigrok (which is GPLv3+) the binary as a whole is GPLv3+.

Reflect that in the COPYING file, the documentation, and the GUI.
win32: Use -mwindows to avoid opening a "DOS box".
Bring the cursors into view when they are shown
Added an accelerator for show/hide cursors