- validate nets and respective drivers like everything else
 - load the netlist earlier (in the user scripts) to make it available while
   loading settings
 - add a FIJI::Netlist parameter to respective FIJI::Settings's functions
 - add a new type 'driver' for FIU elements to distinguish its string
   representation from nets'
 - add new net validation function to FIJI::Netlist
 - use this to make net and FIU validation more generic to simplify FIJISettingsViewer
 - refine and correct handling of the tool phases
 - distinguish fatal errors, missing fields and less important anomalies when
   validating settings depending on the phase etc
 - notify user about validation problems in more detailed messages and tooltips
 - refine FIJI::VHDL to not load settings all the time and receive parameters
   directly instead of via hashes
 - use a global constant ONLY_LOAD in fiji_setup instead of a local variable
 - add a 'show_warning' function to ease opening warning dialogs
 - various other usability/GUI-related improvements
 - allow the deletion of nets