FdoSecrets: Improve client executable path handling (#6915)

* Fixes #6459 

Improves the overall handling of FdoSecrets showing client executable paths to the user. It does the following:

* Check executable file existence as described in [RFC] fdosecrets: add optional confirmation to secret access (#4733)
* Show application PID and dbus address in the client list
* When the executable file is inaccessible, depending on where the client name is shown:
    * when shown inline, e.g. in notification text, where space is limited, clearly say that the path is invalid
    * when shown in auth dialog, show warning and print detailed info about the client
    * when shown in the client list, draw a warning icon

Co-authored-by: Jonathan White <support@dmapps.us>

tests/gui/TestGuiFdoSecrets.cpp

@@ -117,7 +117,9 @@ class FakeClient : public DBusClient
 {
 public:
     explicit FakeClient(DBusMgr* dbus)
-        : DBusClient(dbus, QStringLiteral("local"), 0, "fake-client")
+        : DBusClient(
+            dbus,
+            {QStringLiteral("local"), 0, true, {ProcInfo{0, 0, QStringLiteral("fake-client"), QString{}, QString{}}}})
     {
     }
 };