Fixes #9438

When a relative custom config path is passed via commandline, the logic
which decides if a plugin can be deleted from the plugin manager fails

This reduces problems with old plugins without max version

This excludes other unix systems which was not intended

Its listed as dependency

sys.platform == 'linux' excludeds other unix systems, for example FreeBSD

Its a standard library which will be always available in our builds

Code is also not cross-platform compatible

- sys.platform == 'linux', excludes many other unix systems
- use shutil.which() to test if a command is available

Requires flatpak 1.0 or newer

- Make all menus and shortcuts use the 'app.browse-history' action
- HistoryWindow: Inherit from Gtk.ApplicationWindow

- Show orange warning icon if a field is required
- Show red error icon if validation of the field failed
- Set validation error text as tooltip of the icon
- DataFormWidget emits a signal 'is-valid' on every change

like
The "flags" argument for dialog construction is deprecated. Please use initializer keywords: modal=True and/or destroy_with_parent=True.
or
Using positional arguments with the GObject constructor has been deprecated. Please specify keyword(s) for "title, transient_for, flags" or use a class specific constructor.

See: https://wiki.gnome.org/PyGObject/InitializerDeprecations