BUG: 485829
## Summary
Currently, the plugin just fails silently if the local device is missing the `krfb` package or if the remote device misses an `vnc://` protocol/scheme handler. You click the button and nothing happens.
One issue is, that the plugin is considered `virtualmonitor.available` in the `DeviceDelegate.qml`, even if the check for `krfb-virtualmonitor` fails and no dbus-path is provided. I investigated the behavior a bit, but ignored it in the end as this MR benefits from being shown for device constellations that _could_ provide this feature.
A warning is shown with brief instructions, how to get the plugin working correctly.
- Check if krfb-virtualmonitor is available locally
- Check default scheme handler for vnc:// on device (Linux)
- Show warnings / reasons, if no connection could be established
## Test Plan
Regarding if the devices have mentioned packages installed, we should see different behaviors.
If the remote device has no VNC client, it can not connect to out server. _A warning should be shown._
If the local device hasn't the `krfb-virtualmonitor` available, the remote device couldn't connect. _A warning should be shown._
If both problems are present, _both warnings should be shown._
If none of these are present, no warning should be shown and we should try to establish a connection.
The connection attempts failed? _A warning should be shown._
b24d629802