Do not show uninstall icon if files are missing, closes #1278.
Merge pull request #1578 from cmfcmf/extension-uninstall

This PR was merged into zikula:1.3 branch.

Discussion
----------

Do not show uninstall icon if files are missing, closes #1278.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | ---
| Fixed tickets | #1278
| Refs tickets  | ---
| License       | MIT
| Doc PR        | ---

Commits
-------

85d647897ad22f78cfde61780375be757fdf0f54 Do not show uninstall icon if files are missing, closes #1278. cmfcmf