UI: dim icons in inactive tabs, similar to text

This makes the properties editor navigation bar less saturated. Internally
the icon theme coloring was refactored a bit to move more towards the button
drawing code.

source/blender/editors/include/UI_resources.h

@@ -384,7 +384,7 @@ void UI_GetThemeColorType3ubv(int colorid, int spacetype, unsigned char col[3]);
 void UI_GetThemeColorType4ubv(int colorid, int spacetype, unsigned char col[4]);
 
 // get theme color for coloring monochrome icons
-bool UI_GetIconThemeColor4fv(int colorid, float col[4]);
+bool UI_GetIconThemeColor4ubv(int colorid, unsigned char col[4]);
 
 // shade a 3 byte color (same as UI_GetColorPtrBlendShade3ubv with 0.0 factor)
 void UI_GetColorPtrShade3ubv(const unsigned char cp1[3], unsigned char col[3], int offset);