[backend] Update to 3.6
[backend] Update to 3.6

- General
  - Fixed deprecation warnings up until Qt 5.15
  - Removed MPL-licensed (BSD-incompatible) code
    - A copy of uchardet-0.0.1, since MPL is not BSD-compatible.
      The system libuchardet is used via pkg-config (configure -uchardet).
    - Mozilla's MPL-rdf_utils.c - QTextCodec is used instead.
  - Improved uchardet & ICU selection and auto-detection.
  - Modernized the codebase to use nullptr, override, and default member init
  - Minor build system fixes
  - Added -(no)make qml configure option
- IrcCore
  - Added IrcConnection::connectionCount to count established connections
  - Fixed UTF-8 handling in IrcMessageDecoder::decode()
  - Fixed IrcProtocol to allow spaces in the PASS command
- IrcModel
  - IMPORTANT BEHAVIOR CHANGES
    - IrcBufferModel has been changed to deliver notice messages to
      the target buffer, and create the buffer if it does not exist.
  - Fixed IrcBufferModel to sort channels with keys before channels without
    keys, and group as many channels into a single join command as possible.
- IrcUtil
  - Fixed IrcTextFormat to exclude semi-colons while percent encoding URLs
  - Fixed IrcTextFormat to retain HTML entities
  - Fixed IrcLagTimer to count unanswered pings towards the lag

communi/communi-sailfish,communi/communi-sailfish,communi/communi-sailfish