Extensions: Update - show current version of extension

[pe7er]

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)

[pe7er]

I've changed the label "Version" to "Update to Version" to make it more clear that its the number of the new version.

[waader]

I have tested this item ✅ successfully on b06292c

👍

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[infograf768]

Nice.
I could test easily on staging for some extensions who do not specify the minor J version.

[infograf768]

I have tested this item ✅ successfully on b06292c

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[zero-24]

@roland-d please decide if this can go into 3.5

RTC Thanks.

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[brianteeman]

I think "Available Version" is better?
"Update to" describes an action not some information

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[joomla-cms-bot]

This PR has received new commits.

CC: @infograf768, @waader

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[pe7er]

Thanks for testing @waader & @infograf768 !

Good point @brianteeman!
I've just changed the language string to "Available Version"

[brianteeman]

Thanks for doing that @pe7er

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[brianteeman]

I have tested this item ✅ successfully on d8deaed

Works as intended thanks

As a small visual improvement if you remove the 10% on the th then you wont get the ugly/inconsistent wrapping on the titles

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[pjwiseman]

I have tested this item ✅ successfully on d8deaed

Nice feature.

Whilst removing the 10% width on the version fields does avoid the inconsistent title wrappings, it does that at a cost of a rather wide field on a wide display. I'd swing either way. Hence, it's a successful test.

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[ghost]

If for some reason the manifest_cache is empty, then this will cause an error.
See for more stable implementation: #8768

[pe7er]

Thanks @nonumber !
I'll try to correct the patch with your corrected code soon...

[wilsonge]

I'm going to close this in favour of the code proposed by @nonumber in #8768