Forwar port from branch to 7.10: Existing sitemenu parameters do not show in the HST configuration editor interface

[Cloned HSTCONFIGEDIT-137 to be ported to trunk]

Sitemenu parameters in the HST configuration editor do not always show.

Reproduction path:

1. Create archetype project
2. Go to Channel Manager, edit channel
3. Go to HST configuration editor
4. Add some parameters to the main/about menu
5. Click on another menu item
6. Click on the about menu item again

Expected:
After step 5:
form is updated with correct values
After step 6:
The paramters are visible in the UI

Actual:
After step 5:
the form still has the same values as the about menu item
After step 6:
No parameters are visible in the UI

Notes:
This seems to be a little glitchy, adding another parameter brings up the existing ones. In my project they sometimes turned up if I first click on a child menu item.