Ratsnest is updated after exporting changes from the module editor to the layout editor.

Introduced SELECTION_CONDITIONS to determine which menu entries should be visible in the SELECTION_TOOL context menu, depending on the selection.

- zone filling algorithm
- printing
- plotting (pdf & gerbers)
- SVG export
- Specctra export
- 3D viewer

Created dialog_dxf_import.h. Cleaned up some unnecessary stuff and fixed file open dialog DIALOG_DXF_IMPORT.

- DIALOG_DXF_IMPORT works with PCB_BASE_FRAME instead of PCB_EDIT_FRAME
- imported items are not immediately added to a BOARD
- imported items are held in a list, instead of vector
- imported items are instantly visible in GAL view
- added DIALOG_DXF_IMPORT::GetImportedItems()
- code formatting

Reference point is selected for copied items. Enabled autopanning for copy & paste operations (module editor/GAL).

Removed MODULE::AddPad().