Add is available to navigation app

[JF002]

Add IsAvailable() to Navigation App

Navigation app now (since #1838) needs 2 images to be loaded from the resources on the external filesystem. This PR adds an enabled field to the Applications struct. This field is true for all applications expect for Navigation which calls Navigation::IsAvailable(). This methods returns true if the 2 files are available in the resources.

The application list disables the application (draws it in grey, disables the touch callback) if the enable flag is not set.

This implementation is similar than the one used by the Watchface Setting app.

Here is the result from the devkit and InfiniSim :

[github-actions[bot]]

Build size and comparison to main:	Section	Size	Difference
text	376768B	-19188B
data	940B	-56B
bss	63420B	0B