Currently, the FindReplaceLogic disables the incremental mode (i.e.,
search-as-you-type) in case the regex search option is activated. This
makes the FindReplaceOverlay, which uses search-as-you-type in all other
situations, behave non-intuitively when the regex option is activated.

In order to achieve consistent, user-expectable behavior of the find and
replace functionality and because of lacking arguments against using
search-as-you-type in regex mode, this change makes the regex search
mode independent from the incremental mode of the FindReplaceLogic. In
consequence, the FindReplaceOverlay always uses search-as-you-type, no
matter which search options are activated.
In addition, explicit test code for that exceptional behavior is removed
or adapted.

Contributes to
https://github.com/eclipse-platform/eclipse.platform.ui/issues/2066

Contributes to
https://github.com/eclipse-platform/eclipse.platform.ui/issues/2646

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/1911