Add Keyboard Shortcuts Window

[gabrbrand]

Is your feature request related to a problem? Please describe.
Currently there are some shortcuts like Alt + 1 / 2 most users don't know and therefore don't use.

Describe the solution you'd like
It would be good if there was a window that listed all available shortcuts.

Additional context

using Gtk 4.0;

ShortcutsWindow help_overlay {
  modal: true;

  ShortcutsSection {
    section-name: "shortcuts";
    max-height: 10;

    ShortcutsGroup {
      title: C_("Shortcuts Window", "Window");

      ShortcutsShortcut {
        accelerator: "<Alt>1";
        title: C_("Shortcuts Window", "Show Installed Page");
      }

      ShortcutsShortcut {
        accelerator: "<Alt>2";
        title: C_("Shortcuts Window", "Show Browse Page");
      }
    }

    ShortcutsGroup {
      title: C_("Shortcuts Window", "Screenshot View");

      ShortcutsShortcut {
        accelerator: "<ctrl>plus";
        title: C_("Shortcuts Window", "Zoom In");
      }

      ShortcutsShortcut {
        accelerator: "<ctrl>minus";
        title: C_("Shortcuts Window", "Zoom Out");
      }

      ShortcutsShortcut {
        accelerator: "<ctrl>0";
        title: C_("Shortcuts Window", "Reset Zoom");
      }
    }

    ShortcutsGroup {
      title: C_("Shortcuts Window", "Application");

      ShortcutsShortcut {
        // Needs to be added
        accelerator: "<Primary>question";
        title: C_("Shortcuts Window", "Show Keyboard Shortcuts");
      }

      ShortcutsShortcut {
        accelerator: "<Primary>Q";
        title: C_("Shortcuts Window", "Quit Application");
      }

      ShortcutsShortcut {
        accelerator: "<Primary>W";
        title: C_("Shortcuts Window", "Close Application Window");
      }
    }
  }
}

[oscfdezdz]

I agree, I had in a local branch the changes made from a while ago, I've rebased and push them to help-overlay branch but it requires #568 to work in the development version as well.

If you want you can open a merge request to that branch, some of the shortcuts you have proposed are missing and the names and groups are better written, otherwise I'll update it soon.