Implement activity creation

[mwoehlke-kitware]

After various discussions, the current thoughts (but open to further discussion) for how activity creation are:

• User activates a control (probably a button similar to 'new track') to create a new activity. This activates a side panel for creating/editing activities. The activity does not yet exist in the internal models.
• User chooses an activity type from all available types.
• User activates a control to add a track to the activity. Track selection may be limited by the selected activity type (probably this is a future feature not implemented initially).
• Initially, the entire track participates in the activity. It is only at this point that the activity is saved to the internal models. (Otherwise, certain actions such as selecting a different activity cancel activity creation with no changes made.)
• Once a track is added, the user may change the start/end times of that track's participation.
• Depending on the activity type, the user may be allowed to add additional tracks. (Again, this may be reserved for a future feature, and just always allowed initially.)

[aashish24]

@matthewma7 @mwoehlke-kitware is this done now? or is there something unfinished?

[matthewma7]

@aashish24 Yes, it is implemented with this PR. Thank you for reminding us, I constantly forget about closing issues.