Throw exceptions by value, catch by reference

This fixes bug #92 where exceptions are not being catch thrown by
cxxopts.

This is also standard practice :)
https://stackoverflow.com/a/2522311/2720026

Fix cmake

* The manpage is tiny, and distributions will
  recompress the manpage into a different
  format anyhow.

* Use `GNUInstallDirs` as it allows changing
  of all dirs in a standardised, Kitware
  supported way. `LIB_SUFFIX` is not idiomatic
  and not supported by Kitware:

    http://kwwidgets.org/Bug/view.php?id=10287

* Use ISO C++11 over the GNU variant, as this
  makes cross-platform porting efforts easier
  to other compilers.
* Don't search for a C compiler, speeds up
  configure step.
* Use proper idiom to change `CMAKE_INSTALL_PREFIX`
  default:

    https://cmake.org/cmake/help/latest/variable/CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT.html

Fix 64 bit install

Added my Fedora COPR (experimental repo)

Today I tried to make SPEC file for slop.

I created a .spec file for slop and set up my COPR for building.  Also I
wrote [a script][1] that can produce the spec file and trigger new build from
sources at GitHub (my fork, for now):

  [1]: https://github.com/AloisMahdal/slop-copr-build

For now my plan is to keep an eye on new releases and re-run the script
manually (the script needs python-copr, which is only available at
Fedora AFAIK).

Should you consider providing "official" spec file, feel free to re-use
mine.

Re-add arrow key adjustment during drags (again)

Also check for an initialized keyboard, fixes #77

Add .gitignore file