Fix hiding of columns for recent Qt versions

To hide QTreeView colums, KCachegrind always used to set
the corresponding column widths to 0. However, this
does not work on recent Qt versions which enforce a
minimum column width by default (ie. columns stay visible).

So better use the correct way of hiding a column by
calling setColumnHidden(). This also should work for
old Qt versions.

Change {Call|Instr|Source}View accordingly.