Bug 729068 - Gimp windows state is lost when closed from the window manager

Set gtk_window_set_skip_taskbar_hint() to FALSE for all dock windows.

(cherry picked from commit 35f8355af486f5a5122c74e1d5b05370d1877c95)
Bug 588512 - Grid Spacing errors depending on unit

Always round grid spacings and offsets to integer precision when
using them for snapping and grid drawing. While we might *want* to
have subpixel precision grids, all the UI is pretending it to be
on pixel boundaries. Make that pretense an fact.

(cherry picked from commit 08c6aefa2398dcd36f0c8963633715de6a6430b4)
app: add gimp_grid_get_spacing() and gimp_grid_get_offset()

and do the rounding to integer there instead of in several places.

(cherry picked from commit 5e2ded502066e7d54a120a0fb03927e41b723453)
Doc NULL return for gimp_display_name() when no GUI.
app: gimp_image_rotate(): fix rotation of sample points

and also notify the image of guide and sample point movements after
rotation. This was probably broken since the introduction of canvas
items.

(cherry picked from commit 21ea6216f72d96a8d900b9da609fdc3f56f687f5)
app: default to GIMP_HELP_BROWSER_WEB_BROWSER if there is no webkit

(cherry picked from commit f1573c42d2ab9a582cdba6f5a1a242e98530531e)