ui: sidebar tab selector uses text labels instead of icons

The two drawn tab glyphs were judged not understandable enough; the
switcher goes to the wide policy with titles only, and the glyphs,
their gresource entries and the icon-naming code are removed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/source/e1608/e1608_panels.cpp

@@ -332,12 +332,10 @@ std::vector<SourcePanel> make_e1608_panels(DataSource& source) {
   SourcePanel ao;
   ao.id = "ao";
   ao.title = "Analog out";
-  ao.icon = "audio-volume-high-symbolic";
   ao.widget = build_ao(src);
   SourcePanel dio;
   dio.id = "dio";
   dio.title = "Digital I/O";
-  dio.icon = "input-dialpad-symbolic";
   dio.widget = build_dio(src);
   return {ao, dio};
 }

src/source/source_kind.hpp

@@ -38,7 +38,6 @@ struct ConfigForm {
 struct SourcePanel {
   std::string id;     ///< stack page name
   std::string title;
-  std::string icon;   ///< icon name for the tab, may be empty
   GtkWidget* widget = nullptr;
 };