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().

Reworked the way of processing events in TOOL_MANAGER class.
Added GetCommandStr() for TOOL_EVENT class.

Introduced a new type of action: TA_ACTIVATE to distinguish events activating tools from common tool actions.