#1123 Fix Regular Expression Testbed output

[Miguel Gimenez]

The Regular Expression Testbed plugin skips empty matches that are perfectly valid, so it looks like there were less matches than expected. See Volker Meyer comment in ticket 1120:

I have regex, that get me the indent part of a string "^( *)Something([a-z]+)".
PCRE decreases the number of matches when the indent part is empty. So the "([a-z]+)" part is sometimes GetMatch(1), sometimes GetMatch(2),

It is not related to PCRE but to this line in regexdlg.cpp:

if (!m_wxre.GetMatch(text, i).IsEmpty())
    ret.Add(m_wxre.GetMatch(text, i));

The attached patch fixes the following:
* Fix plugin output keeping the empty matches
* Test regex validity always, not only when the test string is not empty
* Prepares the plugin for wxWidgets 3.1.6, removing the Advanced line in the choice (in 3.1.6 it will be a synonym of Extended)
* Some refactoring for easier to read code