Allow configuration of builtin main menu buttons

[jmgao]

I got annoyed by the useless notebooks button, so I added a way to remove or configure the label/icons on the builtin menu items.

This config:

menu_item :library    :KOReader           :cmd_spawn          :quiet:exec /mnt/onboard/.adds/koreader/koreader.sh
experimental:menu_main_15505_enabled:0
experimental:menu_main_15505_0_enabled:0
experimental:menu_main_15505_1_label:Books
experimental:menu_main_15505_2_enabled:1
experimental:menu_main_15505_3_enabled:0
experimental:menu_main_15505_default:1

results in this:

(The Home button is removed, but you'll still spawn in there on initial boot: it'd be nice to be able to specify a default, but I'm not entirely sure what the best way of doing so is. Injecting click events in qt seems a lot less trivial than I would expect...)

[pgaskin]

LGTM, thanks!