tmf: Improve SS Explorer filter button description

tmf: Improve SS Explorer filter button description

Change the filter button's tooltip in the State System Explorer from
"Display Only Change" to "Only Display Changes at Selected Timestamp".
This is more representative of what this option does.

Refs bug #419515.

Change-Id: I457627d2ff60b2efee9f8f962a00e883b0a8e63c
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/33153
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Hudson CI