[Build] Correctly choose /usr/lib/i386-linux-gnu as LIBDIR on 32b Debian

QMAKE_HOST.arch is empty on x86
[ToolButton] Use QImage for multiIcon

Instead of 4 separate QPixmaps, use just one QImage and paint only
the current part (according to button state) of the image on button.
[Bookmarks] Use keyword as a shortcut for bookmark url in LocationBar

It is now possible to load bookmarks only with entering its keyword in
LocationBar and then pressing Enter (no need to select the bookmark
from popup completion).