fdo#72125 - Expert config dialog takes too much time to come up

This is not a full fix, this just trims some of the fat in
CuiAboutConfigTabPage::FillItems.
Specifically
  - improve string handling
  - don't use catch/throw unnecessarily

Change-Id: I5b4987407bd007fbbb18f76fcf26f4a33282c0c0
Signed-off-by: Stephan Bergmann <sbergman@redhat.com>:
 reintroduced lost "/" into path of recursive FillItems calls
Do not create paths starting "//" when root is just "/"

Change-Id: If0b413a4fdd93465074548c7ea5451288c1d12aa
More Expert Config Page fixes

Change-Id: Iaa2b7df4246ab3fa31737126f27e4a9da3814048