feat: allow external drop and add dropzone

[skjnldsv]

Drag and drop in current folder (with tall header because of recommended files)
Drop in sub-folder	Narrow height header look

• Allow dropping external files from your OS to Nextcloud
• Add a drop zone with a way to hide it (see Introduce a notice for instant upload / drag&drop in below file list #4483)

Fix #4483

@nextcloud/designers for visual input

[tcitworld]

How does it handle rich workspaces?

[artonge]

Code looks good, but not a fan of how it looks :/
Have we looked at how others are doing it ?

[skjnldsv]

@nimishavijay @jancborchardt please advise updated design in the first comment!

[skjnldsv]

@nimishavijay @jancborchardt please advise updated design in the first comment!

Design tuning can/will be a followup though

[nimishavijay]

Nice! This looks good already, nice touch with the folder opening icon :)

Follow up points:

• While dragging in the header, if your cursor is hovering over the header, there should be some hover feedback, similar to when your cursor is over a subfolder. So the background of the header section can change to --color-primary-light and the font color to --color-primary for example.
• Since this is shown only while dragging, the message could be even more attention grabbing, so we could make the font bold

(ref. Gdrive's drop indication which is really attention grabbing)

[skjnldsv]

• While dragging in the header, if your cursor is hovering over the header, there should be some hover feedback, similar to when your cursor is over a subfolder. So the background of the header section can change to --color-primary-light and the font color to --color-primary for example.

Those two colours doesn't really go together? Is this a mistake? :)