Popup->hide insertions should operate on currently selected sequences, not just the first

The 'hide insertions' option in the popup menu does not preserve insertions amongst selected sequences, only the one that was under the mouse when the popup menu was opened.

Two options are needed:
* Hide Insertions for Sequence
 - current behaviour - suggest this is moved to the 'Sequence' submenu
* Hide Insertions for Selection
 - hides columns gapped in all selected sequences - remains on top-level for convenience, also added to Selection dropdown menu.

Hide insertions for selection when the whole alignment is selected is effectively 'Hide gapped columns' - which will be of use to people working on subsets of alignments where the original column numbering must be preserved.