screenshot: End screencast with same keybinding as start screencast
Currently, the screencast keyboard shortcut shows the UI to begin a
screencast, but there is no way to end it with a keyboard shortcut.
This behavior is inconvenient and seems to be a regression, as a wide
variety of online resources recommend using the same shortcut to start
and end screencasts.
This commit introduces a simple check when the showScreenRecordingUI
function is called by the show-screen-recording-ui keybinding.
If the showScreenRecordingUI function is called while a screencast
is in progress, the stopScreencast function is called instead
of opening the screenshot UI in screencast mode.