Adjustable timeouts for 12864 menus

[oliof]

Currently, 12864 sub menus have a relatively short fixed timeout before the display returns to the main menu.

It would be nice to be able to adjust the general time-out, as well as mark a given sub menu page as "sticky" for work on the machine that requires the occasional adjustment of values via the menu.

[dc42]

@oliof please create a feature request for this at https://github.com/Duet3D/RepRapFirmware/issues.

[oliof]

@dc42 Submitted #975, thanks.