Revert "Disable tab to spaces converter of `TextViewer`"

This reverts commit 541b9e00fb858064684b099f43e045a44421f192.

This commit has the unseen side effect that on copy and paste of text
that contains tabs, the tabs are no longer converted to spaces. This
revert resolves this problem, but forces users to enable tab to space
conversion in the Java formatter.
Automatically convert tabs to spaces

This commit disables the default tabs to spaces converter in the editor
and instead adds one that is configured in the source viewer. This has
the advantage that we don't rely on Java preferences anymore but
consider our own ones.
Merge pull request #776 from sschaef/tab-to-space-conversion

Tab to space conversion