Draft: Modify Weston server decoration new buttons & omitting borders

Modify weston server decoration to be more in line with modern Gnome/KDE CSD borderless decoration.

Hence border has been resized to zero and buttons have been redesigned to bigger and a more flat appearance.

Most tricky part of this change is resize handling of borderless windows. In clients/window.c I've modified the pointer_handle_motion mechanism which seems to be working fine for the native weston apps.

But for xwayland it's broken it seems that resizing is handled differently and I just can't figure out how. If someone more experienced in Xorg could take a look at xwayland resizing that would be great because I'm pretty much stuck there.

Furthermore this Draft PR replaces the existing theme, looking at the changes I think it would be possible to put this change behind compile switch or configuration key. Question is what is most preferred and would it makes sense to have some theme options weston.