Add spacing to legend text

[LorenDB]

After the changes introduced in #52 made left-aligning legend text possible, it became evident that there was no spacing applied between the legend label and the legend indicator. Therefore, I've applied 3px of spacing to remedy the issue.

[winterz]

is this still relevant?

[LorenDB]

Yes, it's still relevant

[iamsergio]

@dfaure-kdab since you reviewed #52 maybe also review this one ?

[dfaure-kdab]

@iamsergio Looks OK (not sure how to test it though). Is the target branch kdchart-3.0 correct, or should it go into master? How's the picking/merging situation between those two branches?

[iamsergio]

Let's try master, since we don't have a test for it