Issue 27724

This is a follow-up to issue 1919.

The format-area UI needs to be enhanced to support usage of ad-hoc fill styles
(that is, fill styles specific to single drawing shapes, not linked to any
presets from global palettes).

Suggested implementation:

2 new buttons in all fill style (color, hatching, gradient, bitmap) lists
(which are visible in drop-downs, list boxes, "Background Color" dockers etc.):
"Custom..." and "Modify presets...".

   * the "Custom..." button would allow to use an ad-hoc fill style
   * the "Modify presets..." button would send to the palette editing dialog.

To clarify what I'm meaning, I'll give an example.

In Writer, let's create an ellipse. Now suppose that I want to fill it with a
blue-purplish gradient (not currently available from the gradients palette).

I want to be able to do the following:

1) Select the ellipse (currently possible)
2) Open the 1st "Area Style/Filling" drop down and change from "Color" to
"Gradient" (currently possible)
3) Open the 2nd "Area Style/Filling" drop down and choose a "Custom..." item at
its bottom (currently not possible - only the predefined gradients are available)
4) In the gradient editing dialog, configure my own ad-hoc custom gradient and
click OK (currently not possible due to the previuos one)

I also want to be able to do the following (continuing after point 2 of the
previous list):

3) Open the 2nd "Area Style/Filling" drop down and choose a "Modify presets..."
item at its bottom (currently not possible - only the predefined gradients are
available)
4) In the gradient editing dialog, configure my own gradient preset, save it to
the palette, and click OK if I want to apply it to the selected shape, or press
"Close" if I only want to create a preset, but not apply it to the selected object.