Gnome: client side decorations

[diegoiast]

The app looks bad on gnome. We might need to implement our own titlebar.

Some projects for inspiration:

1. http://github.com/shenlebantongying/qt6_editable_CSD_demo
2. https://github.com/stdware/qwindowkit

Sub tasks, ideas?

1. Make the tabbar, the qmdiServer the main widget.
2. Add gap at the start/end of the tabbar for window operations
3. Make a button provider for mimicking Windows/KDE/Gnome/macOS.