don't move the x axis when moving vertical scroll

problem happens when x-axis is out of limit, extended zoom selection with rectangle tool

fix y axis scrollbar

that was a mistake made back in 39b23673b4d1

fix get version for the case that there is a mercurial repo but no tag

in such case hg returns "null" tag

add options to disable/enable symbols

also fixes symbols always shown in multi plot mode, issue #5

simplify scrollzoomer, only the x axis needs extra limits

Note: at this point auto-scale is broken, when X scroll is used it disables the
auto-scale on Y axis as well

fix for scaling settings applied wrong

these slots are also triggered when loading settings thus should be ignored properly