display: Don't bother recording the grab timestamp

It's not used anywhere.
display: Init X11 events separately
window-x11: Simplify some grab op code
cursor-renderer: Popups need to set cursors, too

Popups could not set the cursor image, because the cursor tracker would
ignore window cursors if we had a popup active. The correct condition to
check for is already in should_block_wayland. Rename this to the more
sensible name windows_are_interactable, and use it in the cursor tracker.
workspace: Simplify some more grab op code
display: Simplify yet even more grab op code