Document units should not differ from user units for 0.91

[ScislaC]

Roadmap for 0.91: (this bug)
=================
√ Everything should use only pixels as is (mostly) done in 0.48 (including templates). For guides, grids, 3dbox see bug #1400702.
√ The ability to switch document units should be disabled (it's broken in several ways).
√ Font size should be stored in pixels.

It should be possible to avoid overwriting Inkscape version until any necessary conversion is done. If a conversion appears to be needed, a pop-up can ask the author if it should be done.

Roadmap for 0.92: (bug #1389723)
=================
- Non-pixel units are allowed but everything in a document should use the same units including guides and grids.
- (Guides and grids need code to do this.)
- An extension that saves a document in pixels should be added to allow backwards compatability with 0.91.

See also:
=========
Related mailing list threads:
<http://thread.gmane.org/gmane.comp.graphics.inkscape.devel/44423>
<http://thread.gmane.org/gmane.comp.graphics.inkscape.devel/45073>

Related wiki page:
<http://wiki.inkscape.org/wiki/index.php/Units_In_Inkscape>
<http://wiki.inkscape.org/wiki/index.php/Units_In_Inkscape:_Document_Unit_Change>

Original description:
=====================
Per discussion on mailing list relating to Grids/Guides, etc.

[su_v (suv-lp)]

It might be useful to coordinate with the proposed changes (for 0.91 only) in
- Bug #1384915 “viewBox numerical errors caused by default.svg file”
  <https://bugs.launchpad.net/inkscape/0.91.x/+bug/1384915>

[ScislaC (scislac)]

Modified patch (no string changes) for 0.91.x from Johan's commit to trunk r13748. I have yet to do thorough testing with this.

[ScislaC (scislac)]

This supercedes the previous patch. It is a combination of r13748, 13750, & r13751 from trunk backported to 0.91.

[ScislaC (scislac)]

Supersedes (woohoo multiple spellings) previous patch. Added fix from trunk r13755.

[su_v (suv-lp)]

Updated patch to include fix from r13759 (supersedes previous ones).

[ScislaC (scislac)]

Updated to include fix from r13760 (supersedes previous).

[ScislaC (scislac)]

Supersedes all previous patches, this backports revisions 13748, 13750, 13751, 13753, 13755, 13758, 13759, 13760, plus has a units.xml update via email from Tavmjong.

[su_v (suv-lp)]

Todo:
- review non-default templates in 0.91.x, add viewBox if missing (in px based on 90dpi)
- review and test commits for bug #1384915 which haven't been backported yet
- …