doc fix: various spelling mistakes corrected

Various spelling mistakes (and one use of the wrong verb) across a few domains that show up in documentation.

domain	current	suggestion
GtkButton	"Buttons can has a flat appearance"	"Buttons can have a flat appearance"
GtkCenterBox	"pas %NULL"	"pass %NULL"
GtkEditable	"the text to append"	"the text to insert"
GtkFlowbox	"the are equal"	"they are equal"
GtkSelectionModel	"Call this when a the selection changes"	"Call this when the selection changes"
GtkWIndow	"(e.g. the user or the window manager"	"(e.g. the user or the window manager)"