Port changes from Custom Attributes redesign

[GriffinRichards]

Re-enables custom attribute support by porting some of the changes from #584:

• Numbers in the Custom Attribute tables are now displayed with a value spinner
• The "Type" column is removed from the table, as data types are now evident when you look at the value cell. Data types can still be specified when you add Custom Attributes
• Adding keys that will collide with external keys is now prevented with an error message, rather than allowed to silently fail
• Adding empty keys is now prevented, rather than allowing them to disappear on reload
• Editing the Custom Attributes table for an Event now marks the map as unsaved
• Internally, rewrote a bunch of customattributestable because it was handled unusually (the class was only really for Events, and it shared static functions with the Header tab's version of the table; now they fully share the class)

The rest of the changes from #584 for default custom attributes will be handled some other day (supporting them with our current config format is messy, and the changes are incomplete).