added loading screen

src/source_dialog.hpp

@@ -26,4 +26,17 @@ bool choose_source(Config& cfg);
 /// names signals, so it is loaded by the app once a CONFIG has described them.
 void apply_stored_settings(Config& cfg);
 
+/// Puts a small window on screen saying what `cfg` is about to be opened as, and paints it before
+/// returning. Call it after the chooser has answered and *before* the buffer is planned or a
+/// recording is read: both of those run with no main loop, so nothing drawn later than this would
+/// appear until they are over.
+///
+/// Returns having pumped the main context long enough for the window to be mapped and drawn. The
+/// caller owes a `loading_hide()`, which `run_ui()` does once the main window is presented.
+void loading_show(const Config& cfg);
+
+/// Takes the loading window down. A no-op when none is up, so the paths that never show one -- a
+/// source named with `--server` -- need no notion of it.
+void loading_hide();
+
 }  // namespace udpscope