performance: align structures for 64bit platforms

More info about technique:

https://en.wikipedia.org/wiki/Data_structure_alignment

https://stackoverflow.com/questions/2006216/why-is-data-structure-alignment-important-for-performance

Most CPU are already 64-bit.

• mywindow: _xfwmWindow 64 -> 54 bytes
• moveresize: _MoveResizeData 152 -> 144 bytes
• session: _match 128 -> 120 bytes
• device: XfwmEventMeta 32 -> 24 bytes