Change desktop tooltip icon size from 128px to 64px

When trying xfce-test and Xubuntu 21.04, I noticed that the desktop tooltip icons were so huge, though they hadn't been like so in the past and checking the glade file's history didn't bring up any change to this new value.

Should be a simple fix
https://gitlab.xfce.org/xfce/xfdesktop/-/blob/master/settings/main.c#L1805 https://gitlab.xfce.org/xfce/xfdesktop/-/blob/master/settings/xfdesktop-settings-ui.glade#L21 https://gitlab.xfce.org/xfce/xfdesktop/-/blob/master/settings/xfdesktop-settings-ui.glade#L919 https://gitlab.xfce.org/xfce/xfdesktop/-/blob/master/settings/xfdesktop-settings-ui.glade#L921