Don't close dialog when dragging outside of it

[mvandenburgh]

This PR changes the behavior of the Bootstrap modal component to automatically close on mousedown instead of on click.
The latter is the default behavior of Bootstrap, so I attached a new event listener to accomplish this - see code comments for details.

Fixes #140

[manthey]

Great.

How much work would it be to change this behavior for all dialogs when we have the HistomicsUI plugin present? Would you prefer to merge this and then generalize the behavior or modify this PR to generalize it?

[mvandenburgh]

Great.

How much work would it be to change this behavior for all dialogs when we have the HistomicsUI plugin present? Would you prefer to merge this and then generalize the behavior or modify this PR to generalize it?

I don't think it would be too much work, I can modify this PR to generalize it.

[mvandenburgh]

@manthey I generalized the mousedown behavior for all dialogs with an id of #g-dialog-container. Alternatively we could use .modal, which would apply this behavior to any Bootstrap modal, but without knowing more about how Girder 3's styling works, I went with the more limited scope - let me know if the latter would make more sense

[manthey]

@manthey I generalized the mousedown behavior for all dialogs with an id of #g-dialog-container. Alternatively we could use .modal, which would apply this behavior to any Bootstrap modal, but without knowing more about how Girder 3's styling works, I went with the more limited scope - let me know if the latter would make more sense

Great -- if one of the users asked for it to apply to Girder dialogs, we'll handle it then.