Add visual cues for Units/Buildings that can't be built because of a lack of resources

[Rampoina]

It would be nice to have visual cues for units/buildings that can't be built. The icon should be displayed in red and disabled. Additionally on hover the missing requisites should be displayed differently (in red for example)

[pavanvo]

@Rampoina The game already shown red icons for lack of resources on dev branch, so we need to implement: only on hover the missing requisites should be displayed differently, right?

[Rampoina]

@pavanvo not always, the buildings are not displayed like that, many times the units either. It's not very consistent And while it's displayed in red, it's never disabled.