Summary: A simple widget to execute commands from android desktop

Test Plan:
Add the widget and execute commands from it.
By default, the selected device is the first device in the device list.
The selected device can be changed via the widget title.
If there is no commands, the widget show an empty list
If there is no connected device, show the message located in @string/unreachable_description

Reviewers: albertvaka, nicolasfella, #kde_connect

Reviewed By: albertvaka

Subscribers: kdeconnect, nicolasfella

Tags: #kde_connect

Differential Revision: https://phabricator.kde.org/D12507