Add support for using relative expressions in move and resize actions

Have MoveResizeTo use config_parse_gravity_coord instead of duplicating it locally
Allow MoveResizeTo positions and sizes and per app positions to be relative to screen size
Rename to config_parse_relative_number so it can be used for sizes too
Add relative numbers to width/height in MoveResizeTo
Add relative numbers to MoveRelative
Add relative numbers to ResizeRelative, these are for the client size, not screen size