Filer sidebar consistency with the sidebars of other applications

[grahamperrin]

Spun off from https://github.com/helloSystem/ISO/issues/121#issuecomment-770388224

Generally, aim for consistency across applications so (for example) volumes that appear in the sidebar of Filer should also appear in the sidebar of Image Viewer:

… Filer should not define its own bookmarks but to use those from the Qt File Open/Save dialog. And I tend to agree. …

[probonopd]

This is where the Qt ones from the File open/save dialog are apparently stored:

FreeBSD% cat ~/.config/QtProject.conf 
[FileDialog]
(...)
shortcuts=file:, file:///home/user, file:///home/user/Desktop
(...)

This is where apparently Filer bookmarks currently end up being saved:

FreeBSD% cat /home/user/.config/gtk-3.0/bookmarks
file:///Applications Applications
file:///Applications/Utilities Utilities
file:///Applications/Preferences Preferences
file:///home/user/Downloads Downloads
file:///usr/home/user/testcase testcase

That FIler saves stuff in a Gtk place is of course unwanted.

[probonopd]

Reported upstream at https://github.com/lxqt/libfm-qt/issues/698.

[probonopd]

Right now, the bookmarks in the file manager are in sync with the bookmarks in Gtk file open/save dialogs:

But NOT in sync with the bookmarks in Qt file open/save dialogs:

Maybe it would be best to save the bookmarks in both locations, for Gtk and Qt applications.

But upstream has closed the ticket unfortunately.

[probonopd]

Upstream will not fix this because apparently https://github.com/lxqt/lxqt-qtplugin overrides their Qt file open/save dialogs with custom versions that use the Gtk bookmarks. Getting rid of Gtk is apparently not their goal, but it is ours.

So we will have to do implement it on our own. Volunteers?