Reintroduces Icons for opening/closing the replace area of the overlay.

fixes eclipse-platform#1960

Fixes: eclipse-platform#1977

- The icons are part of a local toolbar. Therefore move them from the
  "obj" folder to the "lcl16" folder.
- Fix file name for high resolution icons for select_prev and
  select_prev icons
- Make more clear what's the file name of and icon and what's it's key

Fixes: eclipse-platform#1977

If command is null a NPE occurs even though from the code it seems this
should be handled by a fallback.

This replaces Optional.of with Optional.ofNullable to account for that
case.

* remove unnecessary casts

Returns functionality whereby open projects _and_ non-project resources get the 'refresh' contextual menu (option was removed for non-project resources via the fix @ eclipse-platform/eclipse.platform#876 ).

TriggeredOperations.getTriggeringOperation() can return null after
TriggeredOperations.remove()

eclipse-platform#2004

…e-platform#1919

Replace the FocusListener with a ShellAdapter which will remove the
search scope as soon as the overlay is left.
The FocusListener didn't listen to the case where the focus is in the
search bar and the search bar is left (and the overlay is left
implicitly)

fixes eclipse-platform#1919

…orm#1910

Makes the Find/Replace Overlay options accessible by tabbing through the different option buttons.
Implements a new "AccessibleToolBar" class which wraps the ToolBar and
allows for being natively accessible by using the normal traversal mechanism provided by SWT.

fixes eclipse-platform#1910