Date/time preview; tabular date/time formats; template file updated; other changes

* Date and time preview window
* Date/time formats distinguish between regular and tabular text (fixed width format)
* Updated template dictionary for several languages
* Warning displayed to avoid editing system keys in non-template files
* Changed invisible character markers
* Changed date/time system text keys for more hierarchy
* On importing, the "add culture" decision can be remembered for all following cultures
* For date/time, the format can be returned instead of the formatted value
* Fix: The comment is saved also when the primary culture has no text for a key
* Fixed crash when the cursor is at an unknown Unicode character
* Prevent loading mixed template and non-template files