Allow to double-click to accelerate crop and red-eye modifications

Summary:
Currently double-clicking will switch to fullscreen when a tool like
Crop or Reduce Red Eye is used. It would be handy to allow
to accept the modifications without having to tediously position the
cursor over the corresponding button each time.

By using double-click to accept just like it's done for Spectacle's
Rectangular Region capture mode, users can improve the efficiency of
their primarily mouse-based workflow. In particular fixing multiple red
eyes, e.g. in a group picture, will become much faster. In GIMP's
Crop, clicking also accepts (albeit a single click is enough).

Triggering fullscreen can still be done before opening the tool, with a
shortcut or with the toolbutton, and for Crop when clicking
outside the selection. Furthermore, there is the possibility to
Undo if the double-click was unintentional.

To make users aware of the feature, for Reduce Red Eye the inline
help text is changed (at the same time removing the to-be-avoided "you"
in the string). For Crop the docbook will be adapted, since there
is not enough space in the UI for a subtle inline pointer (in a later
patch).

Test Plan:
Double-click in the Crop and Reduce Red Eye tools after
modifying the image. Gwenview should apply the changes immediately.

Reviewers: Gwenview, ngraham

Reviewed By: Gwenview, ngraham

Subscribers: muhlenpfordt, ngraham

Differential Revision: https://phabricator.kde.org/D14755