This patch adds a new Editing-Mask type to layers. This kind of mask
is used to restrict changes on the content bitmap only to areas where
the mask is drawn. The intensity of a pixel change is controlled by the
alpha-value of the mask.
Furthermore a function to invert and clear masks has been introduced.
When a new mask is created for a layer the edit mode of the layer is
also changed to Mask so that the user can immediately start to draw the
mask.
Holding shift while using the polygonal select tool now constrains the
line angle in 22.5 degree increments. This matches the behavior of the
line tool.
Previously, all keydown KeyEvents were accepted, causing parent widgets
not to receive them. With the addition of shortcut handling to keydown,
shortcuts were not called when the ImageEditor was focused.
