ui/ncurses: Add plugin menu and nc-plugin screen

ui/ncurses: Add plugin menu and nc-plugin screen

Add a second pmenu accessible via the main menu which displays
uninstalled and installed pb-plugins. Uninstalled options can be
selected to trigger pb-plugin to install them, after which they are
updated and marked as installed in the menu.
Installed plugins can be investigated by entering the new plugin screen,
where plugin metadata and executables are displayed. Executables can be
run from this screen via cui_run_cmd().

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>