Show detailed information why an effect cannot be loaded

Effects can specify their minimum requirements in their
desktop file:
* OpenGL
* OpenGL 2 (GLSL required)
* Shaders (either ARB or OpenGL 2)

The configuration module uses this information in combination
with which backend KWin is currently using. So if e.g. OpenGL
is used and an effect requires OpenGL 2 a detailed error
message can be showed that OpenGL 2 is required.


FIXED-IN: 4.9.0
REVIEW: 104847