This also adds ImageRegistry, a SVG cache, a bunch of utility functions
for shared pixmaps in CalamaresUtilsGui and renames several qrc files
in viewmodules.

Module descriptors are now module.desc, no configuration allowed inside.
Module config files are <modulename>.conf, installed in
share/calamares/modules.
settings.conf is read from /etc/calamares, then share/calamares, and if
running with --debug also in the current dir.
Module config files are read from /etc/calamares/modules, then
share/calamares/modules, and if running with --debug also in
src/modules/<modulename> relative to the current dir.

Also improved output.

Added ViewManager.
Added dummy Settings class.
Added dummy plugin interface (UI plugins only).
Added dummy greeting plugin.
Added DLLEXPORT macros for UI plugin interface and plugins.