menuconfig: Add .config loading dialog

.config files can now be loaded from within the menuconfig interface.
Show-all mode is turned on if the selected menu entry becomes invisible.

Unrelated fix: The menu path at the top of the display no longer
includes implicit (indentation-based) submenus, which makes it a bit
less spammy and confusing.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>