Add a "focus mode"

[qiyunzhu]

Let the user press a key or button to hide everything but the selected contigs. This will le the user concentrate on the current bin (or a random set of contigs).

Currently there is a workaround: select contigs => invert selection => mask (press "Delete"). This will mask everything else. Then one can work on the selected contigs. When done, press "Z" to undo the last masking to reveal other contigs.

The shortcoming of this approach is that, the user may want to mask some contigs among the focused contigs. The "undo" function can only undo things sequentially. So one can't unmask the all other contigs without unmasking the masked focused contigs first.

One possible solution is to create a separate mechanism from mo.masked to enable focus.