gui2/tchat_log: Add log search support

The search done using the Filter box matches text on timestamps, nicks,
and messages in a case-insensitive fashion.

Some GUI layout adjustments still need to be done for this dialog to
display correctly. Not all of them are possible, unfortunately (e.g.
GUI2 doesn't support setting a minimum cell height yet).
gui2/tbutton: Add 25x25 left/right_arrow_short_ornate variants
gui2/tchat_log: Use 25x25 short arrow buttons for pagination

This should save us some space. More adjustments still need to be done
for this dialog to display correctly under certain circumstances.