pdf-import: text position corrections

at least for rotation of 90 degree
pdf-import: disable bogus image cache

neither 'ref' nor  'str' work as unique key. Minimal test must include
at least render-test.pdf which does not work with both.

Also initial stab on alpha support. Not ready yet, more member functions
need implementation.
PyDia: add read-only Sheet object and update docs

 - PyDiaObjects.dia: add and connect Sheet and MenuItem
 - allsheets.py: Self Documentation Series - Part VII : All Sheets
Bug 732077 - Flowchart object are not resizable (by default)

Change default back to the old default, only fit the text when needed.
[gtk-deprecated] Stop using GtkRuler regardless of GTK+ version

DiaRuler is now always used, not only when GtkRuler is deprecated.
Minor glitch with some numbers fixed, redrawing slightly optimized.
More tool-tips for actions
dia_log_message(): improve time stamp readability
Avoid line number overflow in text_distance_from()
Bug 733344 - Zigzagline has wrong arrow head if placed without the "zag" part

arrows.c: don't move arrow if it would change direction
autoroute.c: distribute points even for straight lines to have room for
arrow movement (non zero line length)
[substitute] remove Standard object specific "Convert to Path" again

At least for "Standard - Box" and "Standard - Bezier*" there is no user
visible gain anymore with the object specific implementations. All their
functionality is integrated with DiaPathRenderer and global menuc entry
"<Selection>/Convert to Path" now.
path: Fix DiaPathRenderer::draw_polygon() to create continuous path

also fix some typos/dix while add it
path: add binary path operations to <Selection> context menu

If there is more than one object selected the context menu offers extra
entries: Union, Difference, Intersection and Exclusion. These are implemented
based on DiaPathRenderer and new path_combine() function. The latter still has
some issues, but it's good enough to check with another compiler/build system.