widgets.standard: add a MainWindowMixin

Signed-off-by: David Aguilar <davvid@gmail.com>
qt: add create_action_button() and use it to build the dock titlebars

Signed-off-by: David Aguilar <davvid@gmail.com>
qt: allow creating titlebars without stretch

The new DAG uses this.

Signed-off-by: David Aguilar <davvid@gmail.com>
defs: add no_spacing

Signed-off-by: David Aguilar <davvid@gmail.com>
model: do not notify when ref is unchanged

Signed-off-by: David Aguilar <davvid@gmail.com>
dag: use dock widgets to simlify the UI

Make things simpler and yet more flexible by removing unneeded
elements (e.g. the "Display" button) and using dock widgets.

Move the graph controls to the graph dock titlebar and the text
fields to the Log dock titlebar to make the layout more
compact.  Replace the splitters with a standard QMainWindow +
dockwidget setup.

Signed-off-by: David Aguilar <davvid@gmail.com>
Merge pull request #183 from davvid/dag

Make the DAG UI simpler and more flexible

Signed-off-by: David Aguilar <davvid@gmail.com>