Fix spelling and punctuation errors in comments and documentation

Fix several occurrences of misspelling "trivial" as "trival"; use "from"
rather than "than"; add missing articles and commas.

Closes https://github.com/wxWidgets/wxWidgets/pull/874

interface/wx/systhemectrl.h

@@ -59,7 +59,7 @@ template <class C>
 class wxSystemThemedControl : public C
 {
 public:
-    /// Trival default constructor.
+    /// Trivial default constructor.
     wxSystemThemedControl();
 
     /**