navigation: fix greying out the app icon if not enabled

[mmind]

Commit 0aead42 ("navigation: Add is available (#1847)") added the ability to draw the app icon in grey and in a disabled state when some prerequisits were not met. Only the Navigation app was using this mechanism due to its icons being stored in the external memory and possibly missing.

Commit 63e0c4f ("Application selection at build time") broke this by always setting the state as true:
for (const auto& userApp : userApps) {
apps[i++] = Screens::Tile::Applications {userApp.icon, userApp.app, true};
}

Fix this by creating an isAvailable() strcuture in the app classes, similar
to how the Watchfaces handle the same problem of checking availability.

[github-actions]

Build size and comparison to main:

Section	Size	Difference
text	379204B	224B
data	944B	0B
bss	22536B	0B

Run in InfiniEmu

[NeroBurner]

Fixing it for just one App feels wrong for me. Would you be interested in implementing the same behavior as done with the Watchfaces?

Having a default isAvailable function returning true, and override it for Apps that need it. In our case just Navigation

[mmind]

@NeroBurner sure, I can do that :-) .

I stumbled upon that thing while doing the other navigation stuff and InfiniSim not showing the flags, but also did not have a grasp on how the whole thing works.

So you're definitly right, doing that only for Navigation does not scale :-) and also thanks for pointing out how things should look correctly.

[mmind]

Hopefully that is what you had in mind :-) ... Follows now how the Wachfaces look

[NeroBurner]

I think you got it right. Good job!

[mark9064]

Looks great, thanks for the PR :)