qtutils: add add_action_bool()

Refactor add_action() and add a new add_action_bool().

Signed-off-by: David Aguilar <davvid@gmail.com>
main: allow users to "lock" the tool layout

Add a "Lock Layout" action to the "Tools" menu.  When locked,
dockwidgets can be hidden but not moved.  This prevents accidental mouse
drags from moving the widgets.

Closes #202

Suggested-by: Stanisław Halik <sthalik@misaki.pl>
Signed-off-by: David Aguilar <davvid@gmail.com>
doc: v1.8.6 release notes draft

Signed-off-by: David Aguilar <davvid@gmail.com>
qtutils: move action setup to add_action_bool()

Signed-off-by: David Aguilar <davvid@gmail.com>
dag: allow users to "lock" the tool layout

Teach the DAG about the "Lock Layout" action.

Closes #202

Suggested-by: Stanisław Halik <sthalik@misaki.pl>
Signed-off-by: David Aguilar <davvid@gmail.com>