Replace hardcoded unit convertions with an inkex based method

[jazzynico]

Follow-up to Bug #1594113 "New extension 'Deep Ungroup' includes hard-coded unit conversion based on 90dpi".

To quote ~suv:
"The recently added extension 'Deep Ungroup' (r14926) includes the method '_get_dimension()' (copied from the original Synfig export extension, bundled with Inkscape since 0.91) which hard-codes unit conversion factors based on Inkscape's legacy internal resolution of 90dpi. Likely this should be converted into a more flexible method which retrieves current values somehow via inkex's Effect() class"

Also applied to synfig_prepare.py, and possibly other extensions.