Allow property editing for multiple selected objects

[vanch3d]

This is a request that is slightly different from the issue re: groups, see https://github.com/polotno-project/polotno-board/issues/2

At the moment, you can select multiple objects on a page, drag them around, change position and even resize. But the property editing widgets in the top bar (e.g. colour, font, size, etc.) are not available when more than one object is selected.

It would greatly help design if common properties such as font or colour could be applied simultaneously to all selected objects.

It should be possible to edit every property common to all the selected objects at the same time. For example:

• if all selected objects are text, then all the properties are available and applied to all objects
• if selected objects contain text and image, then only Effects (common to both types) is available

[lavrton]

That is a very good suggestion.

As the first iteration, I want to change multiple objects if they have the same type. Like when you select several text elements, you can change fonts for all of them.

[lavrton]

I think it is already implemented for most of the types.