* Make emote tab completion search for substrings
Previously, tab completion only searched for emotes that start
with the current prefix under the cursor. This commit makes tab
completion look for the current prefix as a substring in any position,
not just the start.
Examples:
* In forsen's channel, tabbing on "pls" will cycle through "DonaldPls", "forsenPls",
"GachiPls", "nyanPls" and "SourPls".
* As a forsen subscriber, tabbing on "sen1" will complete to "forsen1".
* As a pajlada subscriber, tabbing on "shrug" will cycle through
"pajaShrugL" and "pajaShrugR". (Unless you are in a channel with
more "shrug" emotes, of course.)
* Add a setting for prefix and substring completion
This commit adds a setting under the "Miscellaneous" section to allow
user to choose whether they want prefix-only or substring emote
completion.
The QCompleter filter mode and `addString` function are now chosen
according to the user's setting.
* Improve description of emote completion setting
Also reintroduce a blank line that went missing.
This will unfortunately mean losing your commands, but they can be restored by
converting the old commands.txt format into the commands.json file
Fix#372