GtkLabel: fix mnemonic-keyval when use-markup is true

GtkLabel: fix mnemonic-keyval when use-markup is true

To extract the mnemonic key value, the string must contain the
underscore. But when the "gtk-auto-mnemonics" setting is true and when
the Alt key is not pressed, the underscore must not be displayed. The
problem was that the 'new_str' variable was used for both purposes:
extract the text to display, and extract the accelerator character.

When the underscore must not be visible, the underscores were removed
from the 'new_str' variable before extracting the accelerator character.

Now there are two strings, one for each purpose.

https://bugzilla.gnome.org/show_bug.cgi?id=674759