fix width plug-in has different widths for normal and bold

[vovkkk]

Steps to reproduce:
1) Using Ubuntu 12.04 and Tomboy from default repository (currently 1.10.1) or from Getdeb (currently 1.11.5).
2) Make sure fix width plug-in enabled.
3) Type two identical lines, but one is bold (Ctrl+B) and other is normal.
4) Press Ctrl+A, press Ctrl+T.

Expected: Identical width of those lines.
Actual result: Lines with different width.

I tested it with few fonts and the issue always reproduced.
Even if I set a mono-space font (any of them; e.g. Ubuntu Mono) as default it has wrong width with fix width plug-in, ain't it ironic?
Also, have to say that previously there wasn't such issue: I used to fix width with bold for a long time; and I have Tomboy 1.6 installed on a Windows which hasn't such issue.

Illustration: http://i.imgur.com/qlKLY.png