Always show undo toolbutton (!33)

The GtkToolbar has a 'toolbar-style' [1] property that controls whether
the contained GtkToolButtons only display an icon, only text or both
an icon and text [2]. In this last case, the icon and text may be
stacked either horizontally or vertically.

Additionally, a GtkToolButton can be forced to show text by setting the
'is-important' property [3] to true.

Up to Gtk 3.10, the default value for the 'toolbar-style' property was
taken by the 'gtk-toolbar-style' GtkSetting [4] so that a preference
could be set by the user or the distribution. So unlees an application
explicitely set the 'toolbar-style' property on the GtkToolbar or the
'is-important' property on GtkToolButtons, things were controlled by
the distribution defaults or directly by the user.

In Gtk 3.10 the 'gtk-toolbar-style' GtkSetting was deprecated and since
than has been ignored. Starting from Gtk 3.10 unless an application
explicitely sets the properties only icons are shown.

GParted does not set any of these properties.

Given that historically the distribution or the user could set the
preference to show text in GtkToolButtons, GParted does a trick to try
to limit the toolbar width. Specifically, it looks for the size of the
"Resize/Move" translated string. If the size is longer than 14 bytes it

 * Splits the translated string on two lines
 * Does not add the undo GtkToolButton

This way the toolbar takes less horizontal space.

For example, in my local language the undo button is not shown, while
with LANG=C it is shown.

With Gtk3 3.10+ there is no reason to do this, simply because text is
never shown. Remove the condition and always show the undo GtkToolButton.

Also for older systems with Gtk < 3.10 this is not a problem because in
Gtk3 the toolbar shows a dropdown menu for remaining toolitems it can't
display. So having the undo toolitem is not a problem at all.

References:

[1] Gtk3 Reference Documentation - GtkToolbar/toolbar-style
    https://developer.gnome.org/gtk3/stable/GtkToolbar.html#GtkToolbar--toolbar-style

[2] Gtk3 Reference Documentation - Standard Enumerations/GtkToolbarStyle
    https://developer.gnome.org/gtk3/stable/gtk3-Standard-Enumerations.html#GtkToolbarStyle

[3] Gtk3 Reference Documentation - GtkToolItem/is-important
    https://developer.gnome.org/gtk3/stable/GtkToolItem.html#GtkToolItem--is-important

[4] Gtk3 Reference Documentation - Settings/gtk-toolbar-style
    https://developer.gnome.org/gtk3/stable/GtkSettings.html#GtkSettings--gtk-toolbar-style

Closes !33 - Always show undo toolbutton