[CMake] Fix the build_images cmake target.

[ice0]

Generate images same as autotools script does.
Depends on #1131, #1225

[ice0]

When you try to run:
make build_images you will face 3 problems:

1. synfig binary not found.
   CMake command depends on the assumption what binary can be found in your path environment. So if it is not already installed the target will fail. But we do not need it to be installed, because CMake allows to use another targets in add_custom_command function. Need to fix synfig-studio/images/CMakeLists.txt
2. Images placed in the wrong folder. They must be located in share/pixmaps/synfigstudio/.
   You can see the correct location after running ./2-build-debug.sh in the _debug folder.
3. build_images target always rebuilds images even if they already built. But it’s better to fix it in a separate issue/PR. ([CMake] build_images target always rebuild images #1229)

[AYESDIE]

So we plan to use ${SYNFIG_BUILD_ROOT}/bin/synfig for the purpose of build_images? But we can only run that command on the assumption we have built synfig.

[ice0]

So we plan to use ${SYNFIG_BUILD_ROOT}/bin/synfig for the purpose of build_images?

Yes and no.
Yes - because this binary will be running to do the job.
No - becase you do not need to write "/path/to/bin/synfig", CMake have a better solution. Please look here: https://cmake.org/cmake/help/v3.0/command/add_custom_command.html

But we can only run that command on the assumption we have built synfig.

Yes. This is required dependency.

[AYESDIE]

@ice0

They must be located in share/pixmaps/synfigstudio/.
You can see the correct location after running ./2-build-debug.sh in the _debug folder.

sif_icon.png and synfig_icon.png are build in _debug/build/share/pixmaps, so those 2 needs to be placed in output/share/pixmaps for CMake one too, right?

I'm almost ready with a PR for this one, except for the doubt I had.

[AYESDIE]

I don't think it needs to be there. It shows no error on logs when I run it by placing everything inside output/share/pixmaps/synfigstudio considering the changes made in #1232. Except for #1229, I think it's ready. I'll be making a PR.