Restore state of find flags in FindToolWindow.

There was an issue with checkboxes "Case Sensitive", "Whole Word", and
"Regular Expression": their state was properly saved in
FindPluginPrivate::m_findFlags, but was not properly restored in GUI of
FindToolWindow on the next run of Qt Creator. This patch fixes this issue.

Change-Id: I31e1e14a4117fbf4f4d17e82b3fa2b90973be339
Reviewed-by: Konstantin Tokarev <annulen@yandex.ru>
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>