use slide toggle for files app

[BernhardPosselt]

see #8701 for reference

@jancborchardt

[MorrisJobke]

👍 Tested with IE8, Firefox and Chrome

[ghost]

💣 Test Failed. 💣
Refer to this link for build results: https://ci.owncloud.org/job/pull-request-analyser/5347/

[PVince81]

Works 👍
I find the animation a bit slow, but that's not related to this PR directly.

[DeepDiver1975]

failing unit tests are unrelated

[scrutinizer-notifier]

A new inspection was created.

[jancborchardt]

Tested and works 👍

[jancborchardt]

And thank you @Raydiation, good on the standardization! :)

[ghost]

💣 Test Failed. 💣
Refer to this link for build results: https://ci.owncloud.org/job/pull-request-analyser/5375/