Add support for the property "encoding" in the configuration file

[kuzzh]

I found that log4qt currently does not support the encoding property in the configuration file, for example, log4j.appender.rollingFile.encoding=UTF-8, and log4j.appender.console.encoding=UTF-8, etc.

So, this PR adds support for these properties.

[kuzzh]

Something wrong with the Qt 5.14 compilation environment?

Build started
git clone -q https://github.com/MEONMedical/Log4Qt.git C:\projects\log4qt
git fetch -q origin +refs/pull/7[2](https://ci.appveyor.com/project/MeonMedical/log4qt/builds/44054499/job/b559m0y4kaqt5b20#L2)/merge:
git checkout -qf FETCH_HEAD
Running Install scripts
call %CMD_VCVARSALL%
**********************************************************************
** Visual Studio 2019 Developer Command Prompt v16.11.15
** Copyright (c) 2021 Microsoft Corporation
**********************************************************************
[vcvarsall.bat] Environment initialized for: 'x6[4](https://ci.appveyor.com/project/MeonMedical/log4qt/builds/44054499/job/b559m0y4kaqt5b20#L4)'
set PATH=%PATH%;%QTDIR%\bin
qmake
'qmake' is not recognized as an internal or external command,
operable program or batch file.
Command exited with code 1

[kuzzh]

After some investigation, I found that appveyor removed support for Qt 5.14.2 in an update on April 2, 2022, so there is no longer a compiled environment for Qt 5.14. You may need to synchronize and update the appveyor.yml file.

Reference.

1. src/_updates/2022-04-02.md
2. src/docs/windows-images-software.md