tdf#103223 custom palette label and a11y improvements

In the color section of the Area fill tab, i've added a custom palette
label above the add and delete buttons, moved the palette label and
control on the same line and improved the accessibility of various
controls by adding tooltips and associating labels.

Change-Id: I4a8c9963fd23312190ba343dbb1a0db587b13d1e
Reviewed-on: https://gerrit.libreoffice.org/29894Reviewed-by: Yousuf Philips <philipz85@hotmail.com>
Tested-by: Yousuf Philips <philipz85@hotmail.com>