Download files inline

Use content-disposition=inline instead of attachment when clicking on a file. This allows the user to preview/see a supported file directly in the browser while unsupported files by the browser are still presented as a download.

This does not affect the "download" button which uses apps/files/ajax/download.php, so the user can force a download by clicking on the "download" button or right click on the file and select "save as".

This at least fixes #1745 and fixes owncloud/apps#1687, maybe more, and gives the choice back to the user whether to download the file or show it in the browser.