Fix memory leak for battery tooltips

Instead of returning a const string, that is fed into strdup, tooltip
functions are now supposed to return allocated strings. This fixes a
memory leak in the battery tooltip.

This is used instead of simply freeing the memory in the battery
tooltip function, since it also avoids a uselesss strdup().

src/battery/battery.c

@@ -220,7 +220,7 @@ void init_battery()
 		battery_timeout = add_timeout(10, 30000, update_battery_tick, 0, &battery_timeout);
 }
 
-const char* battery_get_tooltip(void* obj) {
+char* battery_get_tooltip(void* obj) {
 #if defined(__linux)
 	return linux_batteries_get_tooltip();
 #else

src/battery/battery.h

@@ -97,7 +97,7 @@ void battery_action(int button);
 gboolean init_linux_batteries();
 void free_linux_batteries();
 void update_linux_batteries(enum chargestate *state, gint64 *energy_now, gint64 *energy_full, int *seconds);
-const char* linux_batteries_get_tooltip();
+char* linux_batteries_get_tooltip();
 #endif
 
 #endif

src/battery/linux.c

@@ -319,7 +319,7 @@ static gchar* power_human_readable(struct psy_battery *bat) {
 	}
 }
 
-const char* linux_batteries_get_tooltip() {
+char* linux_batteries_get_tooltip() {
 	GList *l;
 	GString *tooltip = g_string_new("");
 	gchar *result;