Summary:
Default shortcuts for zoom in ({key Ctrl +}) and out ({key Ctrl -})
already exists but not for going back to 100%.
The shortcut {key Ctrl 0} is widely used by different applications
for zoom to original size. This patch adds {key Ctrl 0} as system
default for `KStandardAction::ActualSize`.

FEATURE: 305702
FIXED-IN: 5.50

Test Plan:
Searched lxr for conflicts of `KStandardAction::ActualSize` and
usage of Ctrl+0 shortcut.
Checked the following applications with set system default shortcut:
* Gwenview
* KAlgebra
* KolourPaint
* KStars (FITS viewer)
* Cirkuit
Only Cirkuit uses Ctrl+0 for "Zoom To Fit" action. I'll create a
patch to solve this conflict.

Reviewers: dfaure, broulik, ngraham, rkflx

Reviewed By: dfaure, ngraham, rkflx

Subscribers: jriddell, rkflx, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D14201