Fix one more dark mode issue

The `MZTextView` was not always setting the text color to
`NSColor.textColor`. This seems to be a bug fixed in later versions of
Xcode but since we still compile on High-Sierra we needed to find a fix.

The problem seemed to be that when the initial `textColor` and
`insertionPointColor` is `nil` (i.e. default color) the text color will
be wrong until the first time a non-empty string is shown from a loaded
value of any file or a search result. After the first color change the
value is correct.

App/src/MZTextView.m

@@ -243,6 +243,11 @@ - (void)objectDidEndEditing:(id)theEditor
 
 @implementation MZTextView
 
+-(void)awakeFromNib {
+    self.textColor = NSColor.textColor;
+    self.insertionPointColor = NSColor.textColor;
+}
+
 -(void)setString:(NSString *)string
 {
     NSDictionary<NSAttributedStringKey, id>* defaultAttribute =