libdeng2: Added EscapeParser

Utility for parsing a text string with ASCII Esc (0x1b) escape
sequences, used for rich text formatting in libdeng2.
Fixed|libdeng2|MonospaceLogSinkFormatter: Filter out escape sequences

Monospace log output does not use style formatting.
Refactor|libgui|Font: Use EscapeParser to set up rich formatting

Also cleaned up the implementation by applying the pimpl idiom.