Add system "Home" shortcuts as default for opening Start Page

Summary:
There's no default shortcut for opening the Start Page.
This patch adds the system shortcuts from {nav System Settings >
Shortcuts > Standard Shortcuts > Home} to Gwenviews `Start Page`
action, which defaults to {key Alt Home} and special {key Home_Page}.

Test Plan:
# Open Gwenview
# {nav Settings > Configure Shortcuts > Defaults}
# Check `Start Page` to show system settings shortcuts

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D13166