Updated external dynamic links menutree with Clip

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | -
| Fixed tickets | -
| Refs tickets  | -
| License       | MIT
| Doc PR        | -

Clip publications are added to the available list of links one can dynamically add/generate in a menutree menu. I have this active currently already in Zikula Core 1.3.6 with the current Clip from Github. 

Of course the future of Clip is uncertain. So maybe this can be discussed before pulled.
Updated MenutreeApi for typo in comments

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | -
| Fixed tickets | -
| Refs tickets  | #1524
| License       | MIT
| Doc PR        |
Updated the MenutreeApi for lowercase method names

The method Content and Clip had uppercase names, adjusted.
Also adjusted MenutreeUtil call, which changed from Core 136 to 137
Merge pull request #1524 from espaan/patch-29

Updated external dynamic links menutree with Clip