Mark future transaction dates as "invalid"

MarkAsInvalid() paints a red border around the BTextControl.
We do that now for dates entered by the user that are in the future.
It's not prohibited - maybe someone's workflow has a use for it,
but we point it out subtly.

Future-dates in the transaction list are using gNegativeColor as
text colour to have them stand out similarly.

src/TransactionItem.cpp

@@ -13,6 +13,7 @@
 #include "CBLocale.h"
 #include "Database.h"
 #include "Preferences.h"
+#include "TimeSupport.h"
 #include "TransactionData.h"
 #include "TransactionLayout.h"
 
@@ -122,6 +123,9 @@ TransactionItem::DrawItem(BView* owner, BRect frame, bool complete)
 	owner->ConstrainClippingRegion(&clip);
 	locale.DateToString(fDate, string);
 
+	if (fDate > GetCurrentDate())
+		owner->SetHighColor(gNegativeColor);
+
 	owner->DrawString(string.String(), BPoint(xpos, ypos - fontFactor));
 	owner->ConstrainClippingRegion(NULL);