Builder: use the native file dialog for files on this computer #342

Andy, 2026-10-15: the Builder's file browser (Add Structure from File, Open, Save As, export of tables) is poor. It opens in /, looks foreign on every platform, and lacks the desktop's bookmarks, recent files and search.

These dialogs derive from ewxGenericFileDialog (OpenCalculationDialog, ImportCalculationDialog, SaveExperimentAsDialog, ExportTableDialog), which is wx's generic dialog extended so it can also browse the data server.

Proposal

• Use the native dialog (wxFileDialog: GTK file chooser, Windows and macOS dialogs) for files on this computer, starting in the last-used folder or the home folder.
• Keep data-server browsing as a separate choice, e.g. a 'From the data server...' entry beside it, still using the old dialog. In local mode it is not needed at all.
• Keep the file-type filters, which come from the CalculationFactory registry, so .gro, .car, .pdb and the rest stay listed.

Applies to Linux, Windows and macOS.