kconfig/lxdialog: Add definitions for mininimum (re)size values

Commit c8dc68ad ("kconfig/lxdialog: support resize") added support
for resizing, but forgot to collect all hardcoded values at one single
place.

Also add a definition for the check for a minimum screen/window size
of 80x19.

[ ChangeLog v3:
  * Rename MENU_{HEIGTH,WIDTH}_MIN -> MENUBOX_{HEIGTH,WIDTH}_MIN
  ChangeLog v2:
  * Rename WIN_{HEIGTH,WIDTH}_MIN -> WINDOW_{HEIGTH,WIDTH}_MIN
  * Mention the check for a minimum screen/window size in the changelog
  * Add a comment above the block of new definitions ]

Signed-off-by: Sedat Dilek <sedat.dilek@gmail.com>
Acked-by: Wang YanQing <udknight@gmail.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>