The FindReplaceDialog does currently not find any results when
performing a "Select All" operation for read-only targets, such as a
console view. The reason is that the initial implementation reused
"Replace All" functionality, including the check for the target of being
editable. While this is reasonable for replace operations, it is not for
select-only operations.

This change enables "Select All" functionality for read-only targets. It
refactors the "Select All" and "Replace All" functionalities to reuse
their common logic why adapting both for their specific purpose. It also
adds a regression test for performing "Select All" operations on
read-only targets.

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/1029