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 System Settings → Shortcuts → Standard Shortcuts → Home to Gwenviews Start Page
action, which defaults to Alt+Home and special Home_Page.

Test Plan:

1. Open Gwenview
2. Settings → Configure Shortcuts → Defaults
3. Check Start Page to show system settings shortcuts

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: rkflx

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