Improvements for the file tree view in the panel popup

Summary:
Varioux fixes and improvents, most important:

• added a right-click context menu for column header with option "show details": if disabled only the file names are shown
• showing folder in tree view is fixed and synchronized with folder in (this) panel view

I would like to make the "brief mode" (=only file names) the default: This is imo what most users would expect and more usable.
If you agree i'll add it.

Commits:

Moved KrFileTreeView from PanelPopup to own files

Fixing micro-second rename on double-click in file tree view

Added brief mode for file tree view

Fix selecting and showing folder when tree view is opened

Save/restore tree view brief mode setting

Minor coding improvements

Synchronize tree view directory with panel directory

Removed unused methods in tree view class

Ah, and a screenshot:

Test Plan:

• switching between "brief" and "details" mode + save/restore
• navigating /folder change

Reviewers: gengisdave, asensi, martinkostolny, Krusader

Reviewed By: gengisdave, asensi, martinkostolny, Krusader

Subscribers: asensi, martinkostolny

Differential Revision: https://phabricator.kde.org/D2367