Some enhancement proposed

[orion-project]

I'd like to propose some enhancement in this cool classes:

1. Ability to silently skip some tags in mm files. When I've tried to make mml in Libre Office Math I've noticed that it stores original formula text in the tag annotation, so it should not be rendered. Currently only this tag is skipped.

2. Because it is possible to use these classes separately from qwt, including of qwt_global.h should be optional. I've made it by definition of NO_QWT that one can define in his own .pro file along with including these sources into project.

3. FormulaView widget is a good for using it as independent widget, not only for test purposes. So I've tried to slightly optimize it to parse formula only once when it's set but not for each repainting. Also original scaling by factor 2 is not seems very useful, so I took him a meaning of 'fit to widget'. When one set setScale(true), formula begins to render on all available widget space excluding some customizable paddings.

4. By means of using FormulaView as ready formula displaying widget, it's good for it to have frame. So I've rebased it to QFrame.

5. Toolbar in main window of test project is slightly adjusted: labels and separators added.