Problem with limitation start position in "Export highlighted to file" dialog

Affected test:

• GUITest_regression_scenarios_test_3451

Short description:
It is possible to set the exported area from "604" and to "604".
But it is not possible to do the same after changing the "to" field.

Scenario:

1. Open "COI.aln".
2. Open the "Highlighting" tab on the Option Panel.
3. Select any sequence as reference, "UGENE" color, and "Agreements" highlighting scheme.
4. Click the "Export" button.
   Expected state: the "Export Highlighted to File" dialog appears. It is possible to set the exported area to (604, 604).
5. Set cursor to the "to" field of the exported area. Change the value to "603". Change it to "604" again.
6. Try to set "from"=604 again.
   Expected state: "from" can be set to 604
   Actual state: 603 is maximum for "from".