Kuickshow updates

[Jonathan Marten]

There was some mention of Kuickshow here a couple of months ago.
Although there are other image viewer applications available I still
find it convenient for quick viewing, just doing that with no
distractions.  Since there is obviously still some interest I've looked
at the code and brought it up to date, in particular:

- Works with either Imlib1 (ancient, which some distros don't provide
  any more) or Imlib2 (the current supported version).  The choice is
  made automatically at build time depending on which library is
  available.  CMake options are available to force either or none.

- Can also be built with no Imlib library, using only Qt for image
  loading and display.  This may support newer image formats (e.g. WebP)
  than Imlib does.  Image modifications (brightness, contrast, gamma)
  are not yet supported without Imlib.

- Does not use raw Xlib operations, so hopefully should work on Wayland.
  X11 is still needed at build time if using Imlib.

- A new "Duplicate Window" action.

- General code clean up, eliminating warnings and using modern coding
  standards.

Currently these changes are pushed to https://invent.kde.org/graphics/kuickshow
in the 'work/marten/modernise' branch.  If you use Kuickshow and can
compile from source, pull this branch and try it out.  Feedback welcome!

Best regards,
-- 
Jonathan Marten                         http://www.keelhaul.me.uk
Twickenham, UK                          jjm2 at keelhaul.me.uk