libdeng2|scriptsys: Added built-in function "floor"

Rounding a number down to an integer.
DSSL: Added the "gui" module for GUI related utility functions

"DSSL" means Doomsday Script Standard Library.
Refactor|libdeng2: Use Qt containers in Record
libgui: Added Font and FontBank
Fixed|libdeng2: Path concatenation when separator is not a slash

QDir can only be used to determine path absoluteness if the separator
is a slash or backslash.
libdeng2|ScriptedInfo: Locating all records with a particular type
Client: Added the default UI style package

Ultimately "defaultstyle.pack" will contain all the elements of the UI
style, including images.
libgui|qmake: Fixed typo