Allow to specify a custom action instead of the star

[raimund-schluessler]

This PR allows to specify a custom action instead of the star icon in the sidebar. Useful e.g. for the Tasks app to toggle the completed state of a task.

Since #1288 introduced quite some changes to the structure, I implemented it on top of #1288. The important commit is f1ea31a. I will rebase this PR here once #1288 is in.

[raimund-schluessler]

#1288 needs to be merged first.

[raimund-schluessler]

Good for review now.

[skjnldsv]

Shall we add an example for the docs?

[raimund-schluessler]

Shall we add an example for the docs?

Fine with me. I will add one.

[raimund-schluessler]

Shall we add an example for the docs?

I added an avatar as tertiary action example. Probably doesn't really makes sense to have an avatar there (unless the sidebar is about a user), but it shows the use of the tertiary action.

[skjnldsv]

I added an avatar as tertiary action example. Probably doesn't really makes sense to have an avatar there (unless the sidebar is about a user), but it shows the use of the tertiary action.

Maybe just add a checkbox?

[raimund-schluessler]

I added an avatar as tertiary action example. Probably doesn't really makes sense to have an avatar there (unless the sidebar is about a user), but it shows the use of the tertiary action.

Maybe just add a checkbox?

Yes, might be more reasonable. I changed it.