Fix Advanced settings crop toolbar sometimes cut off

Summary:
When the Crop tool is started in standard mode and then enabling
Advanced settings the window width is not updated, which can result
in a cut off toolbar.
This is caused by the design of the image tools which are not embedded
by a layout to the image view and therefore not automatically
exchanging layout/resize events.
To solve this SlideContainer::eventFilter() now handles
LayoutRequest events of the content widgets.
Updating the geometry for these events makes some layout problems in
SaveBar visible which is also derived from SlideContainer.
The message and link labels need to be adjusted to be still shrinkable
and don't force the SaveBar getting too wide.

FIXED-IN: 18.08.1

Test Plan:

• Open Crop without checked Advanced settings option
• Set Advanced settings and check if window width is adjusted to show the whole toolbar
• Modify 3 or more images to enable SaveBar and show all possible parts (buttons, info and links)

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D14917