nemo-view.c: Check all selected files before showing "Open With <app>"

[Fantu]

real_update_menus() shows the default application at the top of the context menu ("Open With ") only when every selected file opens in an external application. The loop over the selection read selection->data instead of l->data, so only the first selected file was checked.

Since show_app is also what reset_open_with_menu() uses as filter_default (#2350), a wrong value affects both the top item and the "Open With" submenu.

How to reproduce

1. In Preferences > Behavior, select "Run executable text files when they are opened"
2. In a folder, create a.sh (not executable) and b.sh (executable), sharing the same default application (e.g. gedit), and use the list view so that a.sh is listed first
3. Select both files and open the context menu

Before:

• the top item is "Open With gedit", but activating it opens a.sh in gedit and runs b.sh
• gedit is missing from the "Open With" submenu, so there is no direct way to open both files in it
• with the executable file listed first the menu is correct, so the result depends on the order of the selection

After:

• the top item is "Open" and gedit is listed in the "Open With" submenu, whatever the order
• choosing gedit there opens both files in it, without running anything
• with two non-executable files nothing changes: "Open With gedit" at the top and gedit not repeated in the submenu

Testing

Tested in a VM on Debian unstable with nemo 6.6.4, rebuilt with only this change (the code is the same on master): both orders of the selection, "Open With > gedit" on the mixed selection, and two non-executable files as a regression check.

I found it while checking about Debian bug #946870, the single-file case already fixed by #2350.