Minor FilePicker improvements

The header bar for windows is now a button.
There is a title label indicating what the file picker is doing (set per
render)
Changing the functions on the FilePicker now refreshes the files inside,
which re-animates the icons to show that something is actually changing
when someone launches a File-picking action while they're already using
it.
Fixed FilePicker index-selection clicking

Replaced my manual mouse calculation with simple buttons. The result is
colored, but slightly transparent, backgrounds on all icon titles.
FilePicker button fix

Rearranged button draw order so that the old button could still be
clicked.