Fixed visual artifacts due to a mistake about conversion to std::wstring in Qt6 port.

The library is ported to Qt6, in additions to receiving several fixes, improvements and code cleanups.

Main changes:

• TERM env variable is set to xterm-256color when not set with QTermWidget::setEnvironment().
• getForegroundProcessId() is added to QTermWidget class to allow terminal apps to check if the user has started a process in the shell and alert them.
• A great amount of code cleanup.
• Falcon color scheme is added.

Main changes:

• A problem is fixed in switching the color scheme between dark and light.
• Prevented a crash on splitting the terminal under kwin_wayland.

Main changes:

• Bidi rendering is enabled by default.
• Text DND follows the settings for newline trimming and multiline prompt.
• QTermWidget can be used as a Qt Plugin.

Main changes:

• Handled the zero history size.
• Added API documentation and followed the naming convention in some places of the code.
• Return focus to the terminal on closing the search bar.
• Code cleanup and enhancement.

Main changes:

• Bumped minimum required Qt version to 5.15.
• Fixed crash under (Plasma) Wayland on opening tab and splitting.
• Quote drag-and-drop file names.
• Backported CNL and CPL support from Konsole.
• Some code cleanup.

Main changes:

• Dropped the support for KDE3 color scheme formats.
• Properly implemented the "Action after paste" feature.
• Added modes for background image. How background images are drawn is changed and manual reconfiguration may be needed. See docs/configuration.md for more details.
• Added a method to disable bracketed paste mode.
• Fixed the execution after pasting when Ctrl is hold.

Bumped the version to 0.16.1, for the point release of qterminal.

Main changes:

• Keyboard commands are handled properly (Shift+Up/Down/Right/Left work correctly).
• Dropped some obsolete Qt classes and cleaned up the code.