Add mechanism to push files without reload

[gokr]

Peter is working on a little mini framework enabling more fine grained control over viewers. One simple feature this enables is to be able to "push" (via a button or via modified file notification) a file to the viewer without causing a reload of the application on the viewer. The application code should also be able to get a callback so that it can handle this file in whatever the way it would like to, like for example pushing it further via BLE as an OTA firmware update. :)

[perttikasanenhitseed]

This could be closed. Works in 2.1 alpha5

[gokr]

Works in 2.1.