This PR adds to Extensions: Update the current version of an extension. The current list of extensions that could be updated to a newer version only shows the new version number.

This feature was requested by wkone in Joomla forum http://forum.joomla.org/viewtopic.php?f=708&t=898474. I think that showing the current and new version gives an administrator more information about possible issues: If the extensions uses semantic versioning then 2.1 to 2.2 is a minor update with little issues. An update from 2.9 to 3.1 might give somewhat more issues.

Testing Instructions

I had to test this feature on a Joomla 3.4.5 website because I was not able to use the "Extension: Update" funtion in Joomla 3.5Beta1. After installing some old versions of some extensions in 3.5Beta1 I did not get the list of updates for those extensions. Probably because those extensions do not have new versions for 3.5Beta1...

Edit: Workaround to test this patch on Joomla 3.5Beta1:
manually edit libraries/cms/version/version.php and temporary change the constant for Release & Development level:
const RELEASE = '3.4.5';
const DEV_LEVEL = 'stable';
With these values Joomla will find new extension updates for old 3rd party extensions.

Don't forget to change it back to the original values after testing this patch. Original values:
const RELEASE = '3.5';
const DEV_LEVEL = '0-beta';

Before the PR

I've installed 3 older versions of extensions that use Joomla's update functionality.
Extensions: Update (via Extensions > Manage > Update) shows that there are 3 updates that can be installed. It shows the version of the extension that will be installed but not the current version

After the PR

After installing this PR the Extensions: Update (via Extensions > Manage > Update) shows Current version and Version (that will be installed)