Add Tab key multi-select for non-adjacent sequences in edit mode

[bistace]

Hi,

PretextView already allows users to select adjacent sequences while in edit mode. This PR makes it possible to select non-adjacent sequences.

Here is what was added:

• Adds Tab key to mark/unmark individual sequences for multi-selection while in edit mode
• Selected contigs are highlighted with an orange cross pattern
• Press Space or middle-click to consolidate all marked contigs into an adjacent group
• Clears selection on mode exit, primary click toggle and after consolidation

Here is how it works:

1. Enter edit mode (E)
2. Hover over a contig and press Tab to mark it (press Tab again to deselect)
3. Repeat for any number of sequences
4. Press Space or middle-click to consolidate. The selected sequences are rearranged to sit next to each other, anchored at the position of the first selected sequence and editing mode activates

[additive3]

@bistace
Nice addition to the functionality, thanks.