sidebar: even more cleanup of text panel

Contains the following changes:
- Kill SvxSBFontNameBox.
- Remove ToolboxFontSelectHandler.
- Move .uno:Color to a separate toolbox. It allows to
  remove some hardcoded uno commands from the code, and
  also solves a redrawing issue.
- General cleanup.

Change-Id: I6fb797aa95d1a73dca0003ad1a40aee55e27e163
Reviewed-on: https://gerrit.libreoffice.org/7893
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>