Client|Widgets: Added a top margin for LogWidget

Having some space at the top balances the content better.
Refactor: Renamed to DENG2_STR_ESCAPE() macro
Client|GuiWidget: More generic hitTest() method
Client|Widgets: Resizing the console using the mouse
libgui: Mouse wheel motion types

There are two kinds of mouse wheel motion we are interested in:
free movement for scrolling, and discrete steps for player controls.
Client|LogWidget: Scrolling the history with mouse wheel