Undo support added to TabDialogHook. Refs: #163.
New method "Refresh" is used for synchronizing any ruler model operation with the GUI without
changing the currently displayed ruler or tabIndex.

In addition, conversion from and to RTF supports now decimal tabs.
Note that html(5) and all major browsers don't support decimal tabs, which means there is no need to changed XhtmlExporter.

Signed-off-by: Josef Templ <josef.templ@gmail.com>