KCachegrind: fix for bug 161276

For detail level "normal", we want function nodes in the graph to contain 2 lines of space for the symbol name, and the 3rdline for cost display.

Up to now (and also in KDE3 times), we used hard coded scaling factors of the width/height numbers given by dot.

Thus, the number of lines was dependent on the general KDE font *and* the default font used by dot for space calculation.

Of course, this can be different on every system.

Now we calculate the needed scaling by looking at the general KDE font, such that in "normal" detail level, it is guaranteed that 3 lines fit into function nodes.

In addition, we set a maximal line number of the symbol space so that there is always a line free for cost display.