ENH: Allow saving relative paths to settings

[lassoan]

ctkCoreSettings allows storing paths relative to an "application home" folder. This makes it easy to create fully portable applications (that do not have absolute paths in application settings).

It will be used with this development in 3D Slicer: Slicer/Slicer#5029

[lassoan]

@jcfr This is ready, please have a look. We can set ApplicationHomePlaceholder to use <APPLAUNCHER_DIR> as a basis of relative paths.

[lassoan]

@jcfr if you don't have any further comments then I would merge this so that we can proceed with the Slicer pull request.

[jcfr]

It looks good to me, thanks for address the comment.

Then, within Slicer, I suggest to update these lines by changing occurrences of:

QSettings settings(q->extensionsSettingsFilePath(), QSettings::IniFormat);

into

ctkCoreSettings settings(q->extensionsSettingsFilePath(), QSettings::IniFormat);
settings.setApplicationHomeDirectory(qSlicerCoreApplication::application()->slicerHome());
settings.setApplicationHomePlaceholder("<APPLAUNCHER_SETTINGS_DIR>");

And it look like you did so already 👍