Draft: border-screen: Optional border around screen (#63)

Add option to specify border for screen / window in configuration file.

Can be useful when user turn off window decorations and need a subtle hint or want strong window borders.

This MR simply draws to the surface right before it is deployed. Border is only drawn if setting is set in terminalrc. It draws over existing surface - hence any text etc. - but only 1px? wide + uses CAIRO_OPERATOR_ADD which accumulates layers.

Ref issue #63