Allow multiple and background file opening

Summary:
This revision addresses the following feature :
Open multiple files at once from project view
FEATURE: 395119
https://bugs.kde.org/show_bug.cgi?id=395119

Three features added :
* Opening multiple selected files (Open selected files menu entry)
{F6236568}
* Opening all files in one/many folder(s) (Open selected files menu entry)
* Open a file in the background (Ctrl+DblClick)

Reviewers: #localization, aacid, ltoscano, adrianchavesfernandez, huftis

Tags: #localization

Differential Revision: https://phabricator.kde.org/D15284