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/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;
 };