trunk: ruler position markers incorrect if units ≠ px (regression)

[su_v]

Steps to reproduce:
1) launch inkscape with default prefs, empty new document
2) open 'File > Document properties' and change default units to e.g. 'mm'
3) close document properties
4) move the mouse pointer position on-canvas and watch the marker of the rulers update according to the mouse pointer position

Expected result:
The marker corresponds with the current mouse pointer position.

Actual result:
The position of the marker only is correct if the mouse pointer is near the origin (lower left corner of the page). The further away from the origin the mouse pointer is moved, the more offset the marker in the ruler is.

Based on tests with archived builds, this regression was introduced between r12001-r12004:
- not reproduced with r12001 and earlier revisions
- reproduced with r12004 and later revisions
on
- OS X 10.7.4 with GTK+/X11 2.24.13 and GTK+/Quartz 2.24.14
- Ubuntu 12.10 (64bit, VM), r12063

Changelog for 'src/widgets/rulers.cpp':
<http://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/changes?filter_file_id=ruler.cpp-20091128124040-aej0x7yhxng1m6ly-2338>

[Alex Valavanis (valavanisalex)]

Should be fixed in lp:inkscape r12067. I migrated the position tracking implementation to the much simpler new track_widget behaviour we adopted from Gimp in sp_ruler.