Manage properties

[yohanboniface]

For now, to add or remove a property, one needs to open the table editor, which is not very intuitive:

We want to make it easier to:

• add a new property
• remove a property
• rename a property
• (later) type a property (integer, string, enum…)

This may be done on the layer level, or at the map level (so adding a new property on the map level would add it to each layer).

cf #2337 #2314