Add an optional help text to CheckboxList to explain that resources are missing

[ljahn]

Fixes #2196

This adds an optional help text to the CheckboxList. It is done as an additional argument in the constructor that is nullptr by default. This way, no changes to other code are needed. If a help text is supplied it scrolls along the very bottom of the screen.

If any watch face signals it is not available, the message "Resources missing! Install resource pack to use greyed out watch faces." Is displayed on the corresponding selection screen.

[github-actions[bot]]

Build size and comparison to main:

Section	Size	Difference
text	373088B	272B
data	948B	0B
bss	22536B	0B