Simple dependency management between plugins

[JonasGilg]

The Problem

We want to create a new plugin, that combines data from WMS, WCS and WFS sources that also implements some analysis tools ontop. We currently have a plugin to load WMS data and a WCS plugin is also in the vestec branch.

Currently it is not easily possible to have dependencies between plugins, which initially was also by design, but it would also be code smell to duplicate all this plugin code.

The Solution

Create a small dependency system, that would allow plugins to depend on other plugins.

Things to consider:

• Plugin dependencies have to be declared in the config
• A dependency tree has to be created and the plugins have to be loaded in the correct order
• Plugins should have a headless mode, where the DLL is loaded, but the plugin is not initialized, so they function more like a library

The Alternatives

• Copy and paste the required code to the new plugin
• Put some of the wms/wcs/wfs features into CosmoScout core
• Create some kind of data sharing feature in CosmoScout core

[JonasGilg]

Current idea is, to create a new system for "plugin-libraries" short csl. They will also be situated in the plugins folder. Plugin libraries will have no subclass of Plugin, but can have their own logger. They can just be linked against from other plugins.

[Schneegans]

I think we could move the measurement tools base classes to such a "plugin-library". They are currently only used in plugins.