Added the GTK3 InputStyle abstraction and updated input style/chanview declarations to avoid direct GtkStyle usage in GTK3 builds.

Updated input style initialization and kept GTK2-only RC parsing while applying GTK3 palette styling for the channel tree via CSS-aware paths.

src/fe-gtk/fe-gtk.h

@@ -48,6 +48,15 @@
 #define flag_b flag_wid[7]
 #define NUM_FLAG_WIDS 8
 
+#if HAVE_GTK3
+typedef struct _input_style
+{
+	PangoFontDescription *font_desc;
+} InputStyle;
+#else
+typedef GtkStyle InputStyle;
+#endif
+
 #ifdef HAVE_GTK_MAC
 extern GtkosxApplication *osx_app;
 #endif