Two clicks on file/folder to rename

Summary:
Make renaming of files/folders faster by clicking a second time on the items text to start renaming.
BUG: 205157

Test Plan:
This feature works as follows:

1. select an item by single-click, or one is already selected
2. wait the "double-click-interval"
3. click on the items text
4. none of the cancellations (see below) happens within the double-click-interval
5. inline-renaming starts

Cancellations:
* open any file/folder
* select different item(s)
* start dragging items
* Dolphin loses focus

This feature is just enabled while "Double-click to open files and folders" in system-settings and "Rename inline" in Dolphin are enabled.

Reviewers: #dolphin, #kde_applications, elvisangelaccio, emmanuelp, ngraham, markg, rkflx

Reviewed By: #dolphin, #kde_applications, elvisangelaccio, ngraham, rkflx

Subscribers: rkflx, markg, funkybomber, sars, elvisangelaccio, ngraham

Differential Revision: https://phabricator.kde.org/D7647