lvgl: kconfig: Remove menu and use a 'menuconfig' symbol

The 'Graphical user interface' menu currently contains just the
'LittlevGL Support' symbol and its indented children.

To remove one menu level, remove the 'Graphical user interface' menu,
rename the symbol to 'LittlevGL GUI library' (consistent with e.g.
'Logging' and 'Bluetooth'), and turn it into a 'menuconfig' symbol.

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