Default Style: Added a HiDPI version of the on/off toggle graphic
Default Style: Adjusted the "textaccent" color
Refactor|libgui|Client: Untabbed lines among tabbed lines

It is now possible to set the tab stop to -1 to indicate that the
line should not be tabbed at all. The objective is to allow reseting
the tab stops within a longer piece of content, continuing with a
new set of stops.

Refactored FontLineWrapping and GLTextComposer to allow processing
tabbed lines in distinct ranges.
Client|libgui: Untabbed lines will reset tab stops for following lines

Now inside a longer styled text content, tab stops are only processed
for the tabbed portion of text. The other lines are considered
untabbed and the used tab alignment will reset after any untabbed
line is encountered.

The tab reset escape is _E(`).
libgui|Drawable: Querying if a buffer exists
Client|GL: Composing OpenGL information as a styled text string
UI|Client|PopupWidget: "Info" style for popups

A popup widget may now configure itself for "informational" use.
UI|Client|DocumentWidget: Widget's height defaults to full content height
UI|Client: Show GL information in the About dialog

ContextWidgetOrganizer can now find items' widgets based on the item
label.
Client|Audio|UI: Show audio information in the About dialog
UI|Client|Widgets: ChoiceWidget's width depends on widest child item

Also corrected some workarounds to issues that are no longer relevant,
related to hidden children in a MenuWidget. There is no reason to not
update or otherwise ignore hidden widgets any more.
Merge branch 'master' of ssh://Architecture.local/Users/jaakko/Projects/deng
Refactor|UI|Client: Added rule-based ui::Margins to handle widget margins

Margins need to be rules, too, so that they can be used freely in rule
expressions. Fixed code where an assumption was made that the margin
is the same on all sides of a widget.
UI|Client: Fixed DialogWidget custom actions, ChoiceWidget sizing