Add class QgsComposerRuler
Add composer rulers to composer graphics view
Implement drawing of ruler markers
Fix ruler layout
Don't draw view frame (alignment with ruler widgets)
Fix calculation of ruler y-coordinates
Read composer from xml considers rulers
Better placement for y-coordinate labels
Show ruler marker for cursor position
Store reference to composition in ruler to get page info
Consider multiple pages for ruler scale
Improve coordinate display
Show snap lines in ruler mouse move event
Snap to arbitrary snap lines
Improve handling of snap lines
Implement moving of snapped features together with the snap line
Hide custom snap lines before printing composition
Load and save custom snap lines in composer
Fix ruler shift
Merge composer ruler branch