Changes in support of Journal toolbar

In support of Sugar PR #265 [1] and Sugar Feature [2] to replace the
filter combo boxes on the Journal toolbar with palettes, several
changes/addtions have been made:

(1) A new icon, view-type.svg, to be used on the toolbar for the
    "what" fileter button (Similar to the view-created and view-sort
    icons).
(2) A change to the Gtk CSS to enable palette menu items to display properly
    on top of scrolled windows.

[1] https://github.com/sugarlabs/sugar/pull/265
[2] http://wiki.sugarlabs.org/go/Features/Replace_combo_box_in_journal_search
gtk-toolbar-style = GTK_TOOLBAR_BOTH_HORIZ

We need to use GTK_TOOLBAR_BOTH_HORIZ in order to enable the
set_important method for toolbuttons. This method enables adding a
label next to the button icon.