Implement focus in dialogs

[jasonsanjose]

Our Bootstrap-based dialogs require some improvements to mimic native-like functionality. Some keyboard shortcuts can be supported (see #283), but other keyboard behaviors require focus to work properly:

• Focus "glow" appearance is missing
• Focus is not limited to dialog (currently pressing TAB can change focus back to controls behind the dialog)
• Spacebar should click the focused button
• Modal dialogs should block keyboard shortcuts from being handled by the main app (e.g. global commands in KeyBindingManager)

Twitter rejected a patch to implement focus in modals, see twbs/bootstrap#1720.

[njx]

Since we don't have any dialogs that need focus right now, we should move this to a user story (either combined with the first story that needs text fields in a dialog, or a separate story before that one).

We should also make sure that in the desktop version, we only allow keyboard focus on buttons (on Mac) when the appropriate system preference is set. (Currently it appears that we always allow focus on buttons on both Win and Mac.)

[jlondon]

Added to Backlog - closing.