Configurable separator support

[mbkma]

Now mate-calc supports both period (.) and comma (,) as decimal separators.

fixes #34

[luigifab]

Without the PR with french locale with mate-calc 1.26 debian

• 1.5+2 (decimal = numeric keypad)
• 1.5+2 (decimal = point/dot)
• 1,5+2 (decimal = coma)

---

Without the PR with french locale with mate-calc 1.28 fedora

• 1.5+2 (decimal = numeric keypad)
• 1,5+2 (decimal = coma)
• 1.5+2 (decimal = point/dot)

---

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

• 1.5+2 (decimal = numeric keypad)
• 1,5+2 (decimal = coma)

---

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

• 1.5+2 (decimal = numeric keypad)
• 1,5+2 (decimal = coma)

but when I press the coma, I have a warning:

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

(process:6353): Gtk-WARNING **: 10:26:47.061: Locale not supported by C library.
	Using the fallback 'C' locale.

(mate-calc:6353): Gtk-WARNING **: 10:26:48.358: 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]

warning fixed

[vkareh]

This doesn't work properly for me. Running as LANG=fr_FR mate-calc:

• ✔️ if I type 123,456+1 the result is 124,456
• ❌ if I type 123.456+1 I expect the result to be 123.457, but it's the same as the previous one: 124,456, which is incorrect in both result and display...

I think this PR should take into account the locale, rather than just accepting a comma arbitrarily.

[github-actions]

🔍 Static Analysis Results

Code analysis completed for this pull request.

• Scan-build reports: 1 files generated
• Status: ⚠️ Issues found

The detailed reports are available in the workflow artifacts.