Non-ASCII placeholder text is displayed incorrectly in InputEntry

Translated placeholder text ‘Search’ in the search entry on the toolbar is displayed incorrectly when it contains any non-ASCII characters. It shows a broken character and trailing part of it isn't grayed out. See the attachment showing example in Russian locale where it's pretty obvious (zim 0.65 on linux, gtk 2.24.30, pygtk 2.24.0).

It happens because zim.gui.widgets.InputEntry._show_placeholder_text attempts to apply styles (italic, gray) to whole placeholder text by taking len() of it and using this value as the ending index while pango.AttrStyle expects indices in number of bytes of UTF-8, not Unicode code points. Thus the ending point falls before the end of the text, and often in a middle of a character.