This is basically a continuation of the commits which
refactor code out of the base text editor. For instance,
36fa1de4 and
3a684586.

Also removed the doXXXX() forwarding methods.

Attributes, uniforms, varyings, and constants are shown with a
distinguished icon that is different from regular variables.

The Qt variant is almost exactly the same as GLSL/ES,
so there is little point having a separate variant code.

*.vert and *.frag are now for desktop shaders
*.vsh and *.fsh are now for ES shaders
File/New gives the user the choice which to create

"xyz", "st", and "rgb" are more likely to be what the user
wants than things like "aaar" and "abbb", so sort the
completion list to move common swizzles to the top.

Well, it's not very advanced but it is probably good enough for now.

Don't trigger again the completion when completing identifiers.

Well, this is just a temporary hack :)