Better user experience for the filemanager plugin

[X-Ryl669]

This happens on top of the previous PR that was allowing opening on files on new tab (and via Enter).

This improves the user experience of the file manager in two way:

1. Replace the textual file tree with icons for directories and files (instead of + - and nothing)
2. Allow quick filtering to easy jump to the file to open (in a folder, type "abc" while the manager is opened and the cursor will jump to the first file with base name starting with "abc"), press enter to open it. If you don't type anything for 2s, the previous filtered pattern is forgotten, like usual file explorer on Windows and MacOSX.

This gives this kind of visual changes:

Please notice that there's a bug in micro's lua binding that's independent of this PR, when using time.AfterFunc the callback function's modifications to any buffer and InfoBar isn't refreshed until the next user event. I think it could be solved either generically (by calling screen.Display() in go code after a timer elapsed), or manually, by binding the screen.Display function call as a command that's callable by a lua script). Either method would work.

[lazev]

Hi. Congrats on the enhancements. Is there a chance to open files and folders with a mouse click or double click? At least for me, it doesn't work.