[PATCH] Darken the icons of unavailable buildings/units/technologies

[wraitii]

This small patch darkens (50% grey) the icons of buildings, units and technologies when they are not available (ie the condition to build/train/research them are not them), as well as grayscale them. This makes it easier to tell if something is actually unavailable in case the icon is mostly gray (see, for example, the "Age II" technology. Works using the same overlay as the red "not enough resources" overlay.

[wraitii]

Better version

[wraitii]

possible new overlay for tech pairs

[wraitii]

New patch, which should work properly (tested most stuffs I could think of).
In this version I also slightly lightened the red overlay indicating "not enough ressources", made technologies use the same gradual reddening as the training/construction ones (ie the more resources you lack, the redder it gets).
Changed tech pairs to show a red cross (the png, which should go in ui/sessions/icons) instead of a red overlay (as that could be confusing).
Removed the "selection" overlay on techs as it is thus no longer needed.

Review: the red cross (whether it sits in well or not), and generally if you like the GUI behavior with this patch.
(check for example with the persians and contrast how the "Age II" icon is clearly unavailable while the "persian masonry" or whatever on the right is available, while both lack colors.)

[wraitii]

Committed as of [13212], forgot to change.