Use layershell for dropdown terminal under wayland

[stefonarch]

Use layershell for positioning and layer, similar to lxqt-runner and lxqt-leave dialog windows.

Expected Behavior

Dropdown settings for size are working under wayland and do not show it in taskbars.

Current Behavior

• Position has to be manually fixed for centering and top panel by window rules and will affect all qterminal windows, e.g. <action name="MoveTo" x="80" y="38"/> under labwc.

• Dropdown terminal shows up in taskbar

Possible Solution

Use similar code as we do in runner already: https://github.com/lxqt/lxqt-runner/blob/3deaf15726c59c90f4a53c2d4678799d79889e26/dialog.cpp#L206

Context

Should be among wayland improvements in LXQt 2.1
Related issues are https://github.com/lxqt/qterminal/issues/1028 and https://github.com/lxqt/qterminal/issues/1059.

With a working single mode qterminal -d can be used to reopen the existing dropdown QTerminal.

System Information

• QTerminal version: qt6