don't switch from 'Thread Date' to 'Date' when clicking column header

When sorting by Thread Date, make clicking the Date columm header not switch to sorting by
Date, but remain sorting by Thread Date, just toggling between ascending/desending, and
maintaining the previous behaviour when sorting by Date.
test commit
Remove test
fix bug 3173, 'quick search ignores trailing blank'
Python plugin: Fix ComposeWindow.get_account_selection
add user specified stylesheet option
make the compiler be quiet. patch by Christian Hesse
fix some typos. patch by Andreas Rönnquist