inconsistent cleanup-behaviour when clicking in buttons above table-header.

[kfriedberger]

sometimes the old content does not get removed from the 'contentPane' when loading new content,
and the new content is only appended.

How to reproduce:

• open a current table (like https://buildbot.sosy-lab.org/cpachecker/results/integration-nightly-ldv-unknown/1110.integration-nightly-ldv-unknown.diff.html)
• click on 'Select Columns', then click on 'Select Columns' again.
  The content pane will get longer and longer each time clicking on 'Select Columns'. This also happens when starting with other buttons.

Wanted solution:

• add cleanup code for 'Select Columns'
• check all other buttons that it does not happen anywhere else.
• perhaps refactor the method , such that 'cleanup' is only done in one place.