appDisplay: Reimplement well layout to be width-independent

appDisplay: Reimplement well layout to be width-independent

Use ShellGenericContainer to implement a fully dynamic layout
for the application well.  It's still fixed to 4 columns by default,
but no longer requires a fixed width to be passed in on start.

With another chunk of work, it could likely try to adjust to
the case where we can only fit fewer than 4 items in the well.

Remove the border highlighting on mouseover, since that caused
reallocations, and the grid layout isn't trivial.

Delete the unused shell_global_get_word_with function.