reload collateral resources

**Is your feature request related to a problem? Please describe.**
when developing on _collateral_ resources, like the nav or the footer, reloading (re-previewing) their content is not ideal.
one has to click preview in the word/google doc, which then opens a new page etc.

the reload button of the sidekick in the developer mode is great, but it should be extendable to included content (also content fragments).

**Describe the solution you'd like**
- the cli should record which resources belong to the currently developed page and somehow tell the sidekick.
- the sidekick should offer a drop-down button for all content to be reloaded

/cc @rofe