Added support for changing the object line width

This new setting in the preferences affects the line width used when
drawing rectangle, ellipse and polygon/polyline objects.

Setting the width to 0 will make the line 1 pixel wide regardless of the
zoom level.

Closes #502
Handle object draw order in minimap and save-as-image

Objects are no longer always drawn top to bottom.
Fixed removal of polygon/polyline objects when resizing a map

They were removed when their origin was falling outside of the map. Now
they are removed when their bounding rect falls outside of the map.

Object rotation is not handled yet, however.
Adjust shadow offset to the line width and scale

When the object line width is set to 0 and lines of 1 physical pixel are
drawn, the shadow offset is now adjusted to the scale to avoid a gap
between the line and its shadow.

For line width between 0 and 1 pixels, the shadow offset is now also
smaller to avoid the gap.

Based on feedback at issue #502.
Brought back the actions that open the Properties window

Even though the Properties window is now a dockwidget that can be left
open, some people may prefer to keep it closed and only open it when
needed. The old menu items were re-introduced to make it easier to open
the Properties window in the right context.

This commit reverts parts of 480f1e245d392d52fd, 20eb8cc1de431c8f756 and
d178b4596e8504179b707617.
Merge remote-tracking branch 'upstream/master'