refactor(AModule): hoist format/tooltip resolution to the base module

Move resolveTooltipFormat() (and add resolveFormat()) from ALabel down into
AModule, and add a generic updateTooltip(Gtk::Widget&, ...) helper. This lets
modules that are not ALabel-derived reuse the shared tooltip logic instead of
re-implementing it. Migrate gamemode (the only non-ALabel module reading
tooltip-format) to updateTooltip(box_, ...). ALabel inherits the resolvers.

src/modules/gamemode.cpp

@@ -210,10 +210,7 @@ auto Gamemode::update() -> void {
   lastStatus = status;
 
   // Tooltip
-  if (tooltip) {
-    std::string text = fmt::format(fmt::runtime(tooltip_format), fmt::arg("count", gameCount));
-    box_.set_tooltip_markup(text);
-  }
+  updateTooltip(box_, tooltip_format, fmt::arg("count", gameCount));
 
   // Label format
   std::string str = fmt::format(fmt::runtime(showAltText ? format_alt : format),