Improve keyframe selection

[AndrewPrifer]

Opening this issue since I've heard it from multiple sources now, keyframe selection is currently not as ideal as it probably could be.

Key problems right now:

• Keyframes can only be selected through box selection, not individually
• Box selection is hidden behind a modifier key
• Box selection cannot be initiated anywhere in the dope sheet area, only on a track

Suggestions for improvements:

• Display available modifiers in a status bar (#69)
• Have separate drag areas for the sequence position and the box selection (many examples of this in other software)
• Fix the box selection start area

[akre54]

It would also be nice to be able to use cmd-a / ctrl-a to select all keyframes in a sheet. Often I'm nudging groups of keyframes around and this would be a helpful addition