[gui] Fix menu item labels alignment

Hello, thank you for providing Frama-C.

This MR fixes menu items alignments at the top of the GUI for items with a picture. They are currently centered, which gives them a quite uncommon style:

With my fix, it’s like this:

I wasn’t able to remove the margins on the left and on the right though.

The fix is to change box#add to box#pack when creating the menu items. It is on my fork: https://git.frama-c.com/Frigory33/frama-c

I also removed some unnecessary labeled arguments which didn’t solve the issue.