SFTP filebrowsing

[Yuri Samoilenko]

Hello

Today I'am rewrite kdeconnect-kde sftp plugin to work with sshfs(FUSE)
console utility.
sshfs availability may be checked in sftp plugin settings.

What works:
 - mounting
 - coping file to/from device
 - safe unmouting when error occures or device disconnected

What bad:
 - when we copy large file to device - we can't browse device
filesystem in diffrent app/console/window because of heavy
lags(freezes). As I think lags because sftp server work in one
threaded mode, but I'am not sure.

 - when I'am disabling wifi on device - kdeconnect still mark device
as accessible - e.g. with "green" icon. All plugins still alive - no
destructors called and sftp plugin don't know that termination of
sshfs needed. Any access to mounted filesystem hang until device
disconnected -  about some minutes. May be we can do something like
"heartbeat" pings between devices - to check real availability?

 - external dependency
 - fuse-sshfs in not portable to windows.


To enjoy this feature you must update kdeconnect-android and
kdeconnect-kde to latest commit from filebrowsing branch.