KviCString::getToken(): add option to skip empty elements (default off)

[ctrlaltca]

KviCString::getToken() is used to read option values for font and colors, that are comma separated.
The current behavior of KviCString::getToken() is to skip empty elements if a string contains multiple separators, eg. value1,,value3 would end up in key1=value1, key2=value3, key3=empty.

This is problematic for font options, that can contain empty fields:

"Courier 10 Pitch,10,5,50,,Regular"
family=Courier 10 Pitch pointSize=10 styleHint=5 weight=50 options=Regular stylename= 

As you can see "Regular" ended up in the options instead of stylename.
As a result, the font will be underlined (since Regular contains "u")

This PR adds a parameter skipEmpty, default false, to fix this problem.
There's only one more place in the code using this method, in KviIrcServerParser::parseNumericWhoReply(), where is safer to keep the old behavior (skip any space between the 2 parameters)

Fixes a bug introduced in #2568
re-Fix #2567