digikam: 5.4.0 -> 5.7.0

The build for the version 5.4.0 of digiKam has been broken at the time
prior to this commit, which is the main reason for this update as I
don't think it makes sense to fix the build for 5.4.0 when we're going
to update it anyway.

A lot has changed upstream between version 5.4.0 and 5.7.0 and it's too
much to be summarized here, so here are the URLs to the upstream
announcements:

 * https://www.digikam.org/news/2017-03-14_digiKam_5.5.0_is_released/
 * https://www.digikam.org/news/2017-06-21-5.6.0-release-announcement/
 * https://www.digikam.org/news/2017-09-11-5.7.0_release_announcement/

On the packaging side, we now no longer have the patch that disables
-fno-operator-names because the build runs fine without that patch
(which didn't even apply but I didn't check why) and IMO it doesn't make
sense to rebase that patch for no reason.

Additionally, there were build time dependencies lurking around in
propagatedBuildInputs, which is kinda pointless and the application just
runs fine if those dependencies are listed in buildInputs.

While looking for clues about why that might be necessary I haven't
found any comment about it in the source nor a clarification within the
message of the commit where this has been introduced.

The commit in question is be7b7d90

.

Apart from these changes, the rest is just adding a few dependencies
(kcalcore, libksane, mesa and pcre) to get less errors during
cmakeConfigurePhase.

I've tested digiKam by playing around within a VM using photos I
netcat'ed into it and it works so far. The VM was built using:

nix-build nixos --arg configuration '{ pkgs, ... }: {
  imports = [ ./nixos/tests/common/user-account.nix ];
  environment.systemPackages = [ pkgs.digikam ];
  services.xserver.enable = true;
  services.xserver.displayManager.sddm.enable = true;
  services.xserver.desktopManager.plasma5.enable = true;
  services.xserver.desktopManager.default = "plasma5";
  virtualisation.memorySize = 1024;
}' -A vm

What I didn't test however was whether importing from a camera would
work (as I don't have one), but aside from that, the application seems
to run fine compared to the fact that it didn't even build until now :-)

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @the-kenny, @urkud, @viric, @cillianderoiste, @ttuegel
Cc: @jraygauthier, @fkz, @sh01, @lsix