Set system default shortcut Ctrl+0 for "Actual Size" action

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