Export pano database

[krupkat]

Implement export/import of the pano database.

• save urls to currently opened images and the panos that were detected by the program to a db file
• implement import of the db file, correctly handling missing images

[krupkat]

0. Add new action types e.g. k[Export|Import]Database

1. Add SingleFileOpen in file_dialog.cc with the correct file suffix filter, e.g. .xpanodb

2. Implement RunExportDatabase and RunImportDatabase in the StitcherPipeline

   • std::future<ExportDatabaseResult> RunExportDatabase(const StitcherData &data)
   • std::future<StitcherData> RunImportDatabase(db_path)
   • implementation of the (de)serialization in a separate file in the pipeline directory
   • it makes sense to have the contents of the db file be human readable
   • test in stitcher_pipeline_test.cc

3. Add the "Export database" / "Import database" options in gui (sidebar.cc)

4. Handle the new action types in PanoGui, dispatching calls to RunExportDatabase and RunImportDatabase

This will likely be multiple PRs, e.g. (steps 0 + 1) = 1 PR, (step 2) = 1-2 PR(s), (steps 4 - 5) = 1 PR