kcms/keys: add a "Choose" button to command adding sheet

This makes it easier for users to choose script files, so they don't
have to type out the full path themselves, which is error-prone.

Some code is adjusted accordingly; for example to make `addCommand()` accept URLs (which are what the file dialog gives us) and convert them to local paths, increase the dialog width to accommodate a new button, and rename an existing dialog to make its purpose clear, now that there are two dialogs.

There is no error-checking to make sure that the script file exists, doesn't live at a remote location, is executable, and has a shebang. But those are all pre-existing issues and will be fixed in another MR, probably copying code from the autostart KCM which handles these cases.

## Testing done ##
Tested to make sure that these use cases still worked:
- Add raw command to the text field
- Type local file path into the text field
- Type URL into the text field
- Use the open dialog to select a file


![image](/uploads/933b46aa7a8e68ddf060b93df2a30f02/image.png)

@bharadwaj-raju @teams/usability @teams/vdg