Make sure, a newly saved image is selected in ContextManager

Summary:
After an image is {nav Saved As...} in View Mode, the current image
and selection in thumbnail bar are not updated. As a side effect the
view jumps to the first item in directory instead of showing the newly
saved image. This is caused by D8934.
This patch makes sure, the new image is selected in `ContextManager`
and the window title is updated correctly.

Test Plan:
# Open an image in View Mode
# Use {nav File > Save As} and save image as e.g. "filename2.jpg"
# Image "filename2.jpg" should be viewed and window title updated

* Check that {nav File > Save As} also works correctly after image
  modifications (resize, crop, etc.)
* Check with option {nav Keep same zoom and position} and 100% or
  a custom zoom factor active
* Check current selection while toggling between View/Browse Mode
  after {nav File > Save As}
* Check that {nav Go back to the original} works
* (Some more special tests in D9342)

Reviewers: rkflx, #gwenview

Reviewed By: rkflx, #gwenview

Subscribers: bschiffner, ngraham

Differential Revision: https://phabricator.kde.org/D9342