Change in "Open in new tab" feature in Dolphin

Change in "Open in new tab" feature in Dolphin

Summary:
This patch proposes a change to the "open in new tab" feature.

The "open in new tab" feature will try to open selected items (files or folders) in a new tab, however, if there are no valid items to be opened in a new tab then nothing will happen, making it look like a bug. This patch adds the functionality that when there are no valid items(files or folders) to be opened in a new tab the current folder will be opened.

Test Plan:
1. Select a file(pdf, text, image etc) in Dolphin
2. Click on the "Open in new tab" toolbar button

Expected: since the file is not a valid target to open in a new tab, the current directory should be opened (as is the case where selection is empty)
Actual: Nothing happens after the button is pressed

Reviewed By: #dolphin, elvisangelaccio

Differential Revision: https://phabricator.kde.org/D6182