Bug 516933 – Rewrite attachment UI

Rewrite the attachment UI to better utilize GIO and also to migrate from
GnomeIconList to GtkIconView.  This also introduces a "List View" option
similar to Nautilus, as well as the EAttachmentHandler framework for
extending attachment handling (may eventually replace EMFormatHook).

This commit also fixes a number of secondary attachment bugs:

Bug 311609 – new attachment bar should use regular gtk+ expander
Bug 314923 – Drag and Drop in attachment window is inconsistent and
             requires additional click
Bug 338179 – attachment saving ...
Bug 350364 – Action to get info about attachments
Bug 383047 – Viewing mail attachments
Bug 427235 – Can't copy attachment mime type string
Bug 454091 – Cannot save multiple attachments who have the same name
Bug 494629 – Rethink composer's attachment UI
Bug 553970 – Evolution ignores umask when saving attachments
Bug 577375 – mailto: and attach doesn't URL un-escape