Adapt diff- and merge tool code for PGM and EGit usage

Adapt diff- and merge tool code for PGM and EGit usage

see: https://git-scm.com/docs/git-mergetool

* DiffTools and MergeTools
  * store FS, gitDir and workTree for usage without
git repository (for EGit preferences)
  * add getUserDefinedToolNames() and getPredefinedToolNames()
  * replace getToolNames() with getAllToolNames() that combines the two
lists and put default tool name (diff.tool or merge.tool) as first
element (for EGit preferences)
* FileElement: refactoring of getFile() and friends to have midName
(LOCAL, REMOTE etc.) always added to the temp file name (also for EGit)
* FileElement: added directory attribute that is used in getFile() to
return path with workDir as parent
* DiffTool and MergeTool
  * added errw.flush(), because sometimes stderr is not printed in case
of die()
  * print e.getMessage() always to stderr
* Moved toolname and prompt logic into managers
* Exported internal packages required for egit.ui

Bug: 356832
Change-Id: I71e7f4dc362169a7612ca4f6546a021bc4b2b5f4
Signed-off-by: Andre Bossert <andre.bossert@siemens.com>
Signed-off-by: Tim Neumann <Tim.Neumann@advantest.com>