wayland: Allow setting "no border" flag even without xdg decoration
Historically, noBorder() was used for two things:
- as a substitute for AbstractClient::isDecorated()
- to determine whether the AbstractClient should have a decoration
With async decoration updates refactoring, a few things around noBorder() have changed, which exposed an existing bug in the handling of borderless maximized windows.
It's possible to have a case where an initially maximized window makes an xdg_toplevel.set_maximized request before the initial commit, but creates the decoration object after the initial commit.
Since XdgToplevelClient::userCanSetNoBorder() would return false when maximize() is called in XdgToplevelClient::initialize(), m_userNoBorder won't be updated and therefore the window can end up having a server side decoration.
Previously, it wasn't the case because kwin would do nothing if the decoration is installed and its preferred mode changes after the initial commit but before the surface is mapped. With async decoration fixes, kwin would react as expected, which unfortunately has exposed the bug.
The root cause of the problem is the fact that noBorder() is overloaded, which makes it error-prone.
This patch changes how the noBorder property is treated. Now, it only indicates whether the compositor wants the window to have no borders. If noBorder() is true, it means that the compositor doesn't want the window to have a server-side decoration; on the other hand, if noBorder() is false, it doesn't imply that the window should have a decoration.
Note that, technically, noBorder() can be also applied to the client-side decoration, but XdgToplevelClient::userCanSetNoBorder() forbids that.