GTK3 port

This MR ports planner to GTK3, getting it to build against it and more or less work. It depends on !8 (merged) (and includes commits from it, as built on top of it).

It includes a libgnomecanvas GTK3 port imported from evolution, with some libgnomecanvas classes reintroduced and ported myself that planner still used, which evolution didn't.

Putting this up for any potential additional testing and reporting of problems by others.

Currently known TODOs:

• Gantt chart background color handling (white for workdays); this should be a matter of porting from GtkStyle to GtkStyleContext as a follow-up (lots of currently visible deprecation warnings are about this)
• Ctrl+mousewheel zoom of the chart doesn't work - it is generating GDK_SCROLL_SMOOTH events now on the canvas; needs >=gdk-3.4 to handle
• Custom cell renderer popup handling (as seen with date edits inside treeview) is very problematic without tweaks - moved to #237 (closed) for follow-up
• Solving all GTK related deprecation warnings for gtk 3.0 (dealing with later deprecations is considered subject of follow-up MRs, thus this branch asks deprecation warnings only for things that were deprecated by 3.0 time) - only deprecations left that related to the "Custom cell renderer popup handling" issue
• Solving all newly introduced runtime warnings (primarily some leftover usages of signals that don't exist in GTK3 anymore)
• Dragging new relationship arrows in gantt chart doesn't provide visible feedback while dragging