Additions to updatd terminal emulator execution (D2638)

Summary:
Remove (unused) arguments passed to terminal execution process.

Using the name of the currently selected file as command to execute? How does this make sense?

Added explanation string for new "%d" argument to terminal execution setting

Test Plan:

• tested correct behaviour of "open with"->"terminal" in context menu and "start terminal here" button
• checked string appearing in settings

Reviewers: Krusader, martinkostolny

Reviewed By: Krusader, martinkostolny

Subscribers: martinkostolny

Differential Revision: https://phabricator.kde.org/D2982