Showing all the different errors and warnings when renaming and creating
files/folders without potentially resizing popovers on every keystroke
requires us to know the size of the error messages beforehand, so pack
all of the possible error messages and warnings in labels and those into
a stack. This way we can also neatly crossfade transition between them.
https://bugzilla.gnome.org/show_bug.cgi?id=775636
