New f.label function

Hi,

After dynamic menus, I propose you a new f.label function.

It works like f.title, but is always drawn like an non-selected usual 
menu item.

It could allow to easily emulate some basic dialog boxes like this one:

Menu "reboot"
{
     "Reboot"              f.title
     "Do you really want to reboot?" f.label
     "*No!"                f.nop
     "Yes" ("white":"red") !"shutdown -r now"
}

Or provide some just retrieved information thanks to dynamic menus:

f.dynmenu """
"Info" f.tile
"Battery level $(battery level)" f.label
"Another info $(get_info)"       f.label
"""

Added to the dynamic menus changes: 
https://github.com/maxatome/ctwm-mirror/commit/9549d19a82ecc55da950c7ec7bb9e0753b1897d0

++

Max.