Show 'drop' style hint on column headers for discoverability

[nditada]

There's no indication right now that a pill can be dropped to a column header for filtering. I believe we had a design where a "filter" hint (similar to the 'store' one) would appear when hovering on the table or column header. @jkicillof ?

[bcardiff]

detailed in https://github.com/instedd/mbuilder/issues/325#issuecomment-137537392 a filter text will appear on hover

[asterite]

It now looks like this:

So at least now there's an indication, but the final style should be decided in #325

[mmuller]

Ok in version 1.6-pre4