This trivial diff gets rid of noisy warnings such as
```
./src/common/ignore.c:274:13: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
274 | ignore_load ()
| ^
| void
```
when building with modern clang. This is cosmetic since these functions
are actually prototyped in a header, so if the header is in scope it's
not possible to call them with an arbitrary number of arguments.
Removed GTK2 compatibility shims from headers (fe-gtk.h, gtkutil.h), leaving direct GTK3-facing declarations/includes only (and platform-specific guards unrelated to GTK2 kept as-is).
Cleaned GTK2-specific comments/wording now that behavior is GTK3-native (menu accelerator note and emoji/userlist comment blocks).
Updated GTK helper usages to reference the new ICON_* (and label) macros so GTK3 builds no longer pass stock IDs to button/icon helpers or dialogs, including banlist buttons, DCC windows, rawlog actions, notify dialog/buttons, pevent dialog buttons, tray menu items, and join dialog image helper usage.
Applied gtk_box_set_homogeneous() in GTK3 branches where the GTK2 code requested homogeneous layout (e.g., dialog hboxes).
Updated remaining helpers and setup/preferences UI box constructors (including the shared mg_box_new helper) to follow the same GTK3/GTK2 branching approach consistently across src/fe-gtk/