ability to configure central software installation directory

[hohwille]

Currently every ide installation has its own isolated software folder. This is the way this IDE solution is designed to work. However, for power-users, that may have up to ~100 parallel installations this multiplies the required disc-space per software folder per number of installations even if all IDE installations would share the same versions.

This enhancement shoud allow to configure an optional environment variable DEVON_SOFTWARE_REPO (ideally in ~/devon.properties - see #151). In case that variable is set, it must point to an existing directory. Further, it would change the behaviour of doInstall such that the software is saved to ${DEVON_SOFTWARE_REPO}/${software}/${version} and then symlinked to ${DEVON_IDE_HOME}/software/${software}.
Hence each version of each software tool would physically be only present once on your disc saving lots of disc-space.

However, there are some drawbacks of this feature:

• software that is manipulated (enhanced with plugins/extensions) will cause these manipulations to have side-effects to all projects sharing the same software tool version.
• additional complexity
• issues with symlinks and permissions on windows.

However, as this feature would be optional and not active by default it would still be cool for those that want to use it.

[hohwille]

There is a high danger to confuse this feature with #98.
As #98 already introduced a variable DEVON_SOFTWARE_REPOSITORY we should come up with a different name here to avoid confusion.
IMHO compared to maven #98 is the remote repository while this feature is about introducing a "local repository as cache". Maybe this analogy can be used for documentation, etc.

[hohwille]

Done with PR #155