Improve UI and functionality of proxy preparation page

[nmaarnio]

• [x] Create 3 pages for the unique preparation workflow steps (distance to features, anomaly raster generation, interpolation)
• [x] Improve the 4 workflow UIs and utilize the 3 unique pages in them
• [x] Add functionality to the preparation steps: computation logic, UI interaction, buttons (next, previous, finish, quit), add produced rasters to correct layer groups if setting is on
• [x] Restrict keywords to one (proxy type)
• [x] Map the workflows based on proxy type
• [ ] Add general preparation window that is accessible through settings
• [ ] Change behaviour of custom mineral system: Proxy list is empty and user can add and remove entries. Table has only name column, combobox for workflow and process button

[nmaarnio]

Adding produced raster layers to layer group, general preparation window and custom mineral system will be left for another issue/PR