Attach-to-process dialog: add "Cancel" button, relabel "Ok" to "Attach"

Summary:
Having an explicit button to Cancel instead of just the option to close
the window follows other dialogs.
Labelling the accepting button with "Attach" instead of "Ok" makes it
more obvious what happens on a click, as akso recommended by
https://community.kde.org/KDE_Visual_Design_Group/HIG/Labels

Reviewers: KDevelop, flherne

Reviewed By: KDevelop, flherne

Subscribers: flherne, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4815