Fix: insert action on double click not working as expected

I changed 3 things to make this feature working as expected:

1. if no autocomplete suffix is set in the prefs, it results in a nil argument error. 
This has been fixed.

2. If the experimental setting "darken input textfield colors" is set, the inserted text is in black colour instead of white as expected.
Now the inserted nicknames are in white colour like the rest of the text.

3. Selecting multiple nicknames from the member list wasn't possible due to some unnecessary code.