Replace all operator bool() with operator void*().

* gtk/gtkmm/stockid.[h|cc]:
* gtk/src/iconinfo.[hg|ccg]:
* gtk/src/papersize.[hg|ccg]:
* gtk/src/recentinfo.[hg|ccg]:
* gtk/src/textiter.[hg|ccg]:
* gtk/src/treeiter.hg:
* gtk/src/treepath.[hg|ccg]:
* gtk/src/treerowreference.[hg|ccg]: Use operator void*() instead of
operator bool() because this avoids an implicit conversion to int instead of
bool. This is apparently generally considered a good thing.