Fix 370385: Search for contacts with underscore.

Currently we do word split by underscore. Th result is, that we
cannot search for anything with a underscore. To fix this we
need to add more words to the term list where also the term with
underscore is added. I search in the history of baloo: This handmade
term splitter was added to search for files with underscores (#336034).

So we need either get control over the term splitting, while searching,
or add the term with underscore to terms list. When we want to provide
both (splitted and non-splitted) we also need to take care about the
position parameter.

Without patching xapiantermgenerator we can sill search for the complete
email address on the commandline:

```
cd /tmp/searchplugintest/contacts
$ xapian-delve -t blue -d .
Posting List for term 'blue' (termfreq 1, collfreq 1, wdf_max 1): 101 []
$ xapian-delve -t jack_blue@test.com -d .
Posting List for term 'jack_blue@test.com' (termfreq 1, collfreq 1,
wdf_max 1): 101 []
```
but NOT:

```
$ xapian-delve -t jack_blue -d .
term 'jack_blue' not in database
```

FIXES: 370385
FIXED-IN: 5.17.0