Fixes for invalid NULL sql on date/times

Most files are simple swaps to get rid of extra " as a NULL return will give
invalid SQL if used as follows UPDATE x SET y="NULL";

See github issue 121 for more info.

Extra changes have been made to the following files:
* lib/rdcartslot.cpp
- Added a QDateTime variable to prevent EVENT_DATETIME becoming
  "2016-06-09" "NULL" as it was checked separately for date and time
* lib/rdcut.cpp
- Reworked START_DATETIME and END_DATETIME to save the SQL ifs.  I believe this
  makes it easier to read and understand.
- The >QDate(1900,1,1) and < 8000 is probably not needed but I left it there
  just in case.
* lib/rdescape_string.cpp
- Reworked to add extra " if the date is not NULL see issue 121
* lib/rdfeed.cpp && lib/rdpodcast.cpp
- Had to add a SetRow for QDateTime as with the string conversion the existing
  SetRow would add an extra set of "
* lib/rdsound_panel.cpp && rdairplay/log_traffic.cpp
        && utils/rddgimport/rddgimport.cpp
- Added a QString to contain EVENT_DATETIME to prevent double checks of date
  and time separately (similar to lib/rdcartslot.cpp)
* rdcatchd/rdcatchd.cpp
- Reverted the changes.  The RML here is not touching the DB so is fine, plus
  with the RDCheckDateTime changes you'd have extra " unless it was NULL
* tests/sas_switch_torture.cpp && sas_torture.cpp
- Included missing rdescape_string.h

tests/sas_switch_torture.cpp

@@ -29,6 +29,7 @@
 #include <rd.h>
 #include <rddb.h>
 #include <sas_switch_torture.h>
+#include <rdescape_string.h>
 
 MainWidget::MainWidget(QWidget *parent)
   :QWidget(parent)
@@ -123,7 +124,7 @@ void MainWidget::generateData()
                          SUN=\'Y\',MON=\'Y\',TUE=\'Y\',WED=\'Y\',THU=\'Y\',\
                          FRI=\'Y\',SAT=\'Y\',DESCRIPTION=\"%s\",\
                          CUT_NAME=\"SAS_SWITCH_TORTURE\",\
-                         START_TIME=\"%s\",TYPE=1",
+                         START_TIME=%s,TYPE=1",
 			      SAS_STATION,
 			      SAS_MATRIX,
 			      j+1,

tests/sas_torture.cpp

@@ -29,6 +29,7 @@
 #include <rd.h>
 #include <rddb.h>
 #include <sas_torture.h>
+#include <rdescape_string.h>
 
 MainWidget::MainWidget(QWidget *parent)
   :QWidget(parent)
@@ -137,7 +138,7 @@ void MainWidget::generateData()
                          SUN=\'Y\',MON=\'Y\',TUE=\'Y\',WED=\'Y\',THU=\'Y\',\
                          FRI=\'Y\',SAT=\'Y\',DESCRIPTION=\"%s\",\
                          CUT_NAME=\"SAS_TORTURE\",MACRO_CART=%d,\
-                         START_TIME=\"%s\",TYPE=1",
+                         START_TIME=%s,TYPE=1",
 			    SAS_STATION,
 			    (const char *)desc,
 			    CART_START+j,