Shaders: renamed TextShader to VectorShader.

The code can be used for drawing any vector art, not only font glyphs.
Shaders: vertex VectorShader is implementation independent.

Renamed it to AbstractVectorShader.
Shaders: support alpha in VectorShader::setColor().
Shaders: cleanup of VectorShader.

Using this-> for accessing parent class members breaks KDevelop
autocompletion, using explicit class name instead (but the shorter one).
Shaders: use Int instead of GLint.

Don't know how this only place got forgotten.
Shaders: missing virtual keyword.

This was compiling?! It seems that I don't know more than I think.
Shaders: doc++
Shaders: first DistanceFieldVectorShader implementation.

Currently only fill and outline can be specified. Also crosslinked from
documentation of VectorShader and TextureTools::distanceField().