handle multiple consecutive spaces correctly on paste

[mbkma]

fixes #207

[luigifab]

Without the PR with french locale with mate-calc 1.26 debian & 1.28 fedora

---

Without the PR with c locale with mate-calc 1.26 debian & 1.28 fedora

but when I paste the number with the space, I have a warning:

fabrice@localhost:~$ LC_ALL=c mate-calc

(process:44337): Gtk-WARNING **: 10:33:38.985: Locale not supported by C library.
	Using the fallback 'C' locale.

(mate-calc:44337): Gtk-WARNING **: 10:34:10.211: Invalid text buffer iterator: either the iterator is uninitialized, or the characters/pixbufs/widgets in the buffer have been modified since the iterator was created.
You must use marks, character numbers, or line numbers to preserve a position across buffer modifications.
You can apply tags and insert marks without invalidating your iterators,
but any mutation that affects 'indexable' buffer contents (contents that can be referred to by character offset)
will invalidate all outstanding iterators

---

With the PR with french locale it's working by rebuilding Debian 1.26 source package:

and with c locale, warning on paste:

fabrice@mario:~$ LC_ALL=c mate-calc

(process:7385): Gtk-WARNING **: 10:41:36.675: Locale not supported by C library.
	Using the fallback 'C' locale.

(mate-calc:7385): Gtk-WARNING **: 10:41:37.357: Invalid text buffer iterator: either the iterator is uninitialized, or the characters/pixbufs/widgets in the buffer have been modified since the iterator was created.
You must use marks, character numbers, or line numbers to preserve a position across buffer modifications.
You can apply tags and insert marks without invalidating your iterators,
but any mutation that affects 'indexable' buffer contents (contents that can be referred to by character offset)
will invalidate all outstanding iterators

[mbkma]

these warnings in this pr should also be fixed by 4bff62e